

Counterpoint: X11 wasn’t designed with today’s security needs in mind, and developers were building based on the assumptions that those security holes would remain. We don’t actually want everything that X11 had, we only want the good bits.
Or to put it another way, the switch from X11 to Wayland = https://xkcd.com/1172/

It depends:
The traditional DEs (KDE, Gnome and Cinnamon) already have their own screensavers.
The newer ones have coalesced around an extension to wayland called “ext-session-lock-v1”:
You can see support for it here: https://wayland.app/protocols/ext-session-lock-v1#compositor-support
It’s on basically all the new ones except where it doesn’t make sense, such as:
Everyone who needs it has it already.
There will probably be an ext-session-lock-v2 and get pulled into the traditional DEs at some point, but probably after a whole bunch of getting everyone around the table and in agreement on some security questions: how do we prevent malicious software setting themselves as a screensaver for a screenjacking attack?, what happens when the screensaver crashes?, that kind of stuff…