For example they've removed asyncore, their original loop-based module before the async/await syntax existed. All the software from that era needs a total rewrite. Luckily in debian for now the module is provided as a .deb package so I didn't have to do the total rewrite.
edit: as usual on ycombinator, dowvotes for saying something factually true that can be easily verified :D
And then you answered about downstream code breakage totally outside the std lib.
And the more stuff you pack into the standard library the more expertise you need on the maintenance team for all these new libraries. And you don't want a standard library that is bad, because then people won't use it. And then you're stuck with the maintenance burden of code that no one uses. It's a big commitment to add something to a standard library.
So it's not that things just suddenly break.
I will be forever mad that they did not use that as a breaking opportunity to namespace the standard library. Something like: `import std.io` so that other libraries can never conflict.
Maybe it wasn't very good, but 2to3 was there from the start: