> If Sway, Hyprland, and others each implement their own WM separation protocol

I think that's pretty unlikely. The smaller compositors actually collaborate fairly well, and if sway, hyprland, niri, KDE etc. decide to implement this, I think they will probably work with river to create a standardized protocol that works across compositors. That has happened before. Hyprland is maybe more likely to do their own thing than the other, but if a standardized protocol caught on I think they would follow that.

Gnome though... I don't think there is a great chance they implement something like this even if several other compositors implement a standardized protocol for it.

I feel like the word "protocol" is tripping you up. This isn't meant to be some standard that gets a bunch of traction in other projects. It's a protocol for the the River compositor; as the name suggests. Before this there was, I believe, river-layout-v3. It's all just getting taken to the next level; from layout to full window management.