iOS 12.4.3 now available for some devices that can't upgrade to iOS 13
Apple has released an update of iOS for some older iPhone and iPad models that cannot be updated to iOS 13.2, with the legacy hardware now able to update to iOS 12.4.3.

iOS 12.4 running on an iPhone XS
While the majority of iPhone and iPad users are updating to iOS 13.2 and iPadOS 13.2 on Monday, users of older models not supported by the operating system upgrade are able to download an update to iOS 12. Released without betas or fanfare on Monday, the iOS 12.4.3 update is available to download now on a small selection of older devices.
The list of compatible hardware for the iOS 12.4.3 update includes the iPhone 5s, the iPhone 6 family of smartphones, the first-generation iPad Air, the second and third-generation iPad mini, and the sixth-generation iPod touch.
Older models, including the fifth-generation iPod touch, the iPhone 5c and iPhone 5, and the iPad 4, are currently not able to update, and have to remain on earlier iOS releases at this time.
Apple says that there are security updates in the release. Preliminary testing by AppleInsider suggests that there are some improvements to two-factor authentication on older devices as well.

iOS 12.4 running on an iPhone XS
While the majority of iPhone and iPad users are updating to iOS 13.2 and iPadOS 13.2 on Monday, users of older models not supported by the operating system upgrade are able to download an update to iOS 12. Released without betas or fanfare on Monday, the iOS 12.4.3 update is available to download now on a small selection of older devices.
The list of compatible hardware for the iOS 12.4.3 update includes the iPhone 5s, the iPhone 6 family of smartphones, the first-generation iPad Air, the second and third-generation iPad mini, and the sixth-generation iPod touch.
Older models, including the fifth-generation iPod touch, the iPhone 5c and iPhone 5, and the iPad 4, are currently not able to update, and have to remain on earlier iOS releases at this time.
Apple says that there are security updates in the release. Preliminary testing by AppleInsider suggests that there are some improvements to two-factor authentication on older devices as well.
Comments
Details of the security fixes haven't been published yet but if this follows the pattern of security updates for older macOS versions then iOS 12.4.3 will include a subset of the fixes in iOS/iPadOS 13.2, and possibly some that were already released in iOS/iPadOS 13 through 13.1.3.
using Iphone 6s plus. I'm
trying to update my IOS to 12.4.3. I already downloaded it but I can't install it. Can you help me guys? Thanks a lot.
Maybe. But the iPhone 5 update was absolutely essential to prevent the device from becoming almost unusable. I think it remains to be seen if they will follow-up on that with any actual security patches. Personally, I am doubtful, and I presume that any devices that are stuck on iOS 11 or older should be considered insecure by default at this point.
I'd happily roll back to 12. anything. anything