That was already the case for anything non-trivial in dependencies and language feature adoption. And it was never the case (and still isn't) for projects rely

That was already the case for anything non-trivial in dependencies and language feature adoption. And it was never the case (and still isn't) for projects relying on the standard library and a few popular stalwars (requests, psycopg, pandas, and so on), with conservative adoption of new language features. There's also the fact that 2.7 and before Python seldom added new language systax, whereas 3+ is hell-bent on doing that with every release.