Video: How to downgrade from macOS High Sierra to Sierra
MacOS High Sierra is Apple's newest operating system for Mac, but some users might have issues with applications or home server setups not working correctly. AppleInsider shows you how to downgrade, if you must.
Assuming you've got a mac older than the iMac Pro that works with macOS 10.12 Sierra, there is a way to revert to the older system.
If you're one of those users having trouble and have decided to go back, the first step is to download the Sierra installer. Apple has a convenient support page, with links on how to download the installer from the Mac App Store.

Once the download is complete, you'll see an error saying that the application is too old to run on your machine. But, we can circumvent this by making an external install drive. First, make a backup of all your data, because the rest of this procedure will erase your system drive. Also, if you're on a MacBook or MacBook Pro, now would be a good time to connect to power.

Insert a USB flash drive. When the USB drive shows up in finder, rename it Sierra. Open the Terminal.
In the terminal window, enter: sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Sierra and hit enter.

A little lock icon will show up for user password validation -- just type in your password to authenticate and hit enter.
You will be asked for permission to erase all the data on the drive. Type the letter y, and hit enter. Terminal should now start copying the boot files to the USB drive.
Once the process is complete, you can close the terminal, and eject the USB drive.
Once your data is backed up, click the Apple icon in the upper left corner of the screen, and select Restart. Hold down Command+R, and keep holding the keys until your computer restarts and a loading bar shows up. After it's finished, click Disk Utility, then continue.
You are about to erase your system drive -- make sure you have your backup!
Select your startup disk, which is usually named Macintosh HD. Just formatting the drive won't do, as it will allow only APFS for SSD-based machines.
Instead, hit Cancel and click Partition. Change the format to Mac OS Extended (Journaled), and then click Apply.

Click Partition on the next popup, which will erase all the data on the drive, and warn you accordingly.
When the process is complete, you drive is reformatted to allow a MacOS Sierra installation. Click the Apple icon once again, and Restart the system.
This time, hold the Option button until it restarts to the system selector screen. Plug in your USB installation drive.Once it shows up, click Install MacOS Sierra.

After it loads, click Install MacOS, and continue. Once you've read and agreed to Apple's terms and conditions, click on your main drive. Which we assume is still called Macintosh HD, then click Install.
Continue through all the pop-ups to begin the installation.Once the installation is complete, choose your language, and hit Continue to complete the rest of the installation process.
When complete, open the Mac App Store, and update to the latest macOS Sierra software -- just make sure not to accidentally reinstall macOS High Sierra. This won't turn off notifications to upgrade to High Sierra, you'll still get those daily. Fortunately, they can be turned off permanently.
Assuming you've got a mac older than the iMac Pro that works with macOS 10.12 Sierra, there is a way to revert to the older system.
If you're one of those users having trouble and have decided to go back, the first step is to download the Sierra installer. Apple has a convenient support page, with links on how to download the installer from the Mac App Store.

Once the download is complete, you'll see an error saying that the application is too old to run on your machine. But, we can circumvent this by making an external install drive. First, make a backup of all your data, because the rest of this procedure will erase your system drive. Also, if you're on a MacBook or MacBook Pro, now would be a good time to connect to power.

Insert a USB flash drive. When the USB drive shows up in finder, rename it Sierra. Open the Terminal.
In the terminal window, enter: sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Sierra and hit enter.

A little lock icon will show up for user password validation -- just type in your password to authenticate and hit enter.
You will be asked for permission to erase all the data on the drive. Type the letter y, and hit enter. Terminal should now start copying the boot files to the USB drive.
Once the process is complete, you can close the terminal, and eject the USB drive.
Once your data is backed up, click the Apple icon in the upper left corner of the screen, and select Restart. Hold down Command+R, and keep holding the keys until your computer restarts and a loading bar shows up. After it's finished, click Disk Utility, then continue.
You are about to erase your system drive -- make sure you have your backup!
Select your startup disk, which is usually named Macintosh HD. Just formatting the drive won't do, as it will allow only APFS for SSD-based machines.
Instead, hit Cancel and click Partition. Change the format to Mac OS Extended (Journaled), and then click Apply.

Click Partition on the next popup, which will erase all the data on the drive, and warn you accordingly.
When the process is complete, you drive is reformatted to allow a MacOS Sierra installation. Click the Apple icon once again, and Restart the system.
This time, hold the Option button until it restarts to the system selector screen. Plug in your USB installation drive.Once it shows up, click Install MacOS Sierra.

After it loads, click Install MacOS, and continue. Once you've read and agreed to Apple's terms and conditions, click on your main drive. Which we assume is still called Macintosh HD, then click Install.
Continue through all the pop-ups to begin the installation.Once the installation is complete, choose your language, and hit Continue to complete the rest of the installation process.
When complete, open the Mac App Store, and update to the latest macOS Sierra software -- just make sure not to accidentally reinstall macOS High Sierra. This won't turn off notifications to upgrade to High Sierra, you'll still get those daily. Fortunately, they can be turned off permanently.
Comments
This just seems like a last ditch effort to get back to Sierra with tons of work involved putting the pieces back together.
Apple this is bad.
We've been asked for this, so we delivered.
My two cents is to stay on plain old Sierra unless you have a pressing need to upgrade.
I plan on moving to Final Cut X in the next couple of months, at which point I'll upgrade my Mini. Unless you have compatibility issues, I see no reason to hold off on upgrading.
I prefer the numbering system, myself.
One thing has been going downhill, however, since the beginning of computers generally, including Apple's. Programming has become sloppier. Software used to be highly efficient and exact, and effort was made to make the slimmest, most efficient code possible. That has changed, and there are now all kinds of work-overs, work-arounds, and bits and pieces of left-over or redundant instructions, all of which has resulted in code which is more bloated than it needs to be. Given how much we ask of computers and programs and their integration which we never even dreamed of before, and how much more the hardware itself is capable of, this may a harder task than it used to be. But more effort on the part of today's programmers would still result in a neater, more efficient product.
The new 10.13.3 MDM functionality that now requires DEP to be fully trusted by High Sierra so you can silently install secure kernel extensions without user intervention means you have to change the entire Apple management infrastructure to accommodate that new requirement.
Windows is not immune to these things but Microsoft tends to let enterprise pull their strings and Apple doesn't For example, Apple released Security Update 2018-001 and it's broken several things and we can't fix it until the vendors patch their software. So it ends up being a Catch-22. With Microsoft you can uninstall a security update. You can't do that on macOS High Sierra. You have to backup the Mac, the nuke and pave it and restore the data from backup. That's a time consuming process. Ever re-install Adobe Creative Cloud and all the Apps? Takes for freaking ever.
All that said, Apple does appear to be locking down macOS considerably. SIP, MDM/DEP required, T2 ARM Co-processor Secure Boot in iMac Pro. Eventually the security will be right up there with iOS devices so there will be little need for these security end points in the first place. The MDM will evolve and improve and Profiles will become even more enhanced. The problems will start to just go away.
I've watched on forums and at 3rd-party developer websites as audio production has been affected. High Sierra (especially APFS) wasn't tested and refined enough before release, especially not for it to be put into productivity centers. While you can certainly criticize people for upgrading too soon when they're in mission-critical circumstances, it's well known that Apple is very passive-aggressive about getting people to upgrade (Logic 10.4 just dropped El Capitan and Logic Remote is only purchasable on iOS 11, both pushes being far too premature).
For end-consumers, High Sierra is probably fine. For studios and other workplaces that rely on the stability of their platform, High Sierra has been a serious issue. If you're going to ignore the serious issues in an entire segment of the customer base because they aren't end-consumers, you're cherry-picking the results of High Sierra on the market.