It’s not necessarily sad for GNU tool development to stop (other than bug fixes, of course). I would take that as a sign that they are basically complete.
Not quite in this case. Tmux was started by someone who wanted to update screen with new features but wasn’t able to bend the code that far. I say this not out of spite or meanness, as I used screen for many happy years, but I’d say it’s less complete and more abandoned. It still has maintainers but it seems to me that they’re more “keep the lights on” than actively developing it.
Whether constant development is necessary or not largely depends on the surface area of your tool, both in terms of formal APIs it uses and external data formats and services.
screen didn't even have vertical splits until maybe 5 or 10 years ago. After tmux was already a solidly reliable replacement with vertical splits for years.
