It is already a thing for 2 years, since this is just an update to an old blog post to say that they'll do even more now.

Aside from that, it wasn't a thing, because as per the usual something on the web breaks when you change behavior like that, because some webpages rely on third-party cookies to provide their core functionality.

Someone (in this case the Tor Browser devs) had to come up with a way to have third-party cookies ~~and eat them, too~~ but isolate them from the third-party cookies that got created on other webpages.
On the technical side, this is called "first-party isolation", and basically each domain you browse to gets its own cookie jar to store first- and third-party cookies in.