Full third-party cookie blocking comes to Safari two years ahead of Chrome
Along with iOS 13.4, Safari was updated with improved Intelligent Tracking Prevention to include full third-party cookie blocking and other privacy features.

Safari Intelligent Tracking Prevention has lead the industry in privacy protections
Intelligent Tracking Prevention (ITP) was introduced in 2017 and has gained several privacy features since. Initial reactions to the release were varying as publishers worried about how this would affect cross-site tracking and related data sources for ad sales.
ITP blocked cookies before, but left enough information for trackers to begin tracking users based on what was being blocked. With the newest update, even this type of tracking and fingerprinting is blocked. Google Chrome is expected to have full third-party cookie blocking by 2022.
According to the WebKit Blog, Websites have long used login fingerprinting to track users based on login states in websites. Full third-party cookie blocking prevents websites from seeing information about the "global browser state" which allows them to see what websites you were signed into previously.
No telling yet how this will again affect ad firms, as even with its limitations previously, it was reported that hundreds of millions in revenue were being lost as a result of ITP. Apple has always taken a strong privacy stance with its devices and services, even to the point of world governments pushing for them to weaken security.

Safari Intelligent Tracking Prevention has lead the industry in privacy protections
Intelligent Tracking Prevention (ITP) was introduced in 2017 and has gained several privacy features since. Initial reactions to the release were varying as publishers worried about how this would affect cross-site tracking and related data sources for ad sales.
ITP blocked cookies before, but left enough information for trackers to begin tracking users based on what was being blocked. With the newest update, even this type of tracking and fingerprinting is blocked. Google Chrome is expected to have full third-party cookie blocking by 2022.
According to the WebKit Blog, Websites have long used login fingerprinting to track users based on login states in websites. Full third-party cookie blocking prevents websites from seeing information about the "global browser state" which allows them to see what websites you were signed into previously.
No telling yet how this will again affect ad firms, as even with its limitations previously, it was reported that hundreds of millions in revenue were being lost as a result of ITP. Apple has always taken a strong privacy stance with its devices and services, even to the point of world governments pushing for them to weaken security.
Comments
Edit: with appleinsider.com whitelisted there is an ad between comment one and comment two. Not so when de-whitelisted:
Please explain how the third-party cookies are now blocked. Use iPad Only.
I cleared everything and went to a website. And it is full of Google and F_c_book cookies. I have tracking-off turned on. (Cleared EVERYTHING and reset ad ID), new update, same old cookies.
I long for the old days on my PC where I could specifically block certain sites i.e. Google.
Old iPad running IOS8 had the ability to specifically block 3rd party cookies.
Now there's a service called Scroll where for $5 a month you get a no-ad browsing experience on hundreds of sites and with 70% of revenues shared with the website owner "It’s a future where users don’t get pissed off and journalists don’t get laid off."
Personally I think that's where things are headed for a lot of us. It's not that I'm unwilling to pay for valuable content and services and I hope that pertains to most of you too. I for one just don't want to have 3 dozen subscriptions to cover all the sites that give me the choice of no ads or ad-supported. I'd nearly always choose no ad subscriptions unless it's a website I rarely use. For example I subscribe to YouTube, two music services, Ars and Techcrunch to avoid ads and have a couple of other subscription websites that don't come to mind right off. Services like Scroll would make it easy to manage yet still keep revenue flowing to the content creators so favorite sites don't disappear.
Journalists and writers can't work for free.