> The other reason is that this isn't base functionality, for example an embedded implementation has no need for screenshots.

I'm unsure what kind of graphical environment you can imagine where screenshots are not useful at all (if only to capture the screen when something weird is going on).

There are numerous other graphical environments that Linux runs on where screenshots don't make sense: https://en.wikipedia.org/wiki/Linux_on_embedded_systems

For debugging purposes, the display server can just dump the screen to a file or a serial port itself. The only reason you would need this in the protocol is so other arbitrary applications could capture the screen.

Screenshots on embedded is very useful in some cases, like debugging, or information sharing or archiving, which often performed by smartphone camera today.

Yes, that's why I said you can have the display server dump it to a file. No protocol support is needed for that.

Car navigation panels where Wayland is already used?

Also, screen sharing is not that easy of a problem if done properly (eg. allowing another app to act on the buffer and directing it here and there)