I’ve been on machines in the last few years that didn’t have screen either. Maybe it was a minimal install or something, but I specifically remember having to install it to get some long running stuff going.
(Thinking it was Ubuntu server, but guessing someone will correct me)
Tmux vs screen is an odd one; it kinda feels like screen was included in the era when people were actually trying to make the default install on servers kind of nice to use with a functional set of assumed programs. And now, it is fairly widespread just due to legacy.
Nowadays, and possibly for the better (every line of code is a potential bug and every bug is a potential vulnerability) it seems like systems don’t want to include this sort of stuff. So, I’m sure if the decision were made today, tmux or screen, tmux would win. Unfortunately, “none” seems like the real future option…