One project I'm involved in. We used a dependency which was Python 2 only. When they finally upgraded to Python 3, it was almost a complete rewrite and completely undocumented, and broke a ton of functionality we used. We didn't have the resource to fix the mess and aren't actively developing it, so we dumped it in a Docker container with the pinned versions, and use it as is.
Also worth saying that a lot of commercial C# applications that bundle a Python console are using Python 2.7, because IronPython doesn't go higher than that currently.