There really isn't anything 'legacy' with screen. I've been using it for 20 years daily. I really have no reason to switch to tmux because I really can't see anything screen doesn't already offer me.

Vertical splits were patched in and added a long time ago, screen is usually installed by default, so I know it is there on systems where I'm not allowed to install packages. I can share screen sessions with co-workers, split windows etc.