I will never forget Mercurial's postmortem on their experience with the Python 3 transition. They had very few kind words to say about the process.

https://gregoryszorc.com/blog/2020/01/13/mercurial's-journey...

Part of me even wonders if the transition had any role to play in why Mercurial gradually lost whatever foothold it had in the DVCS ecosystem.

Very interesting article.

I too was rather upset about the Python 2->3 shenanigans and I was working on much smaller projects.