KDE and Gnome already have toggles for it, though Gnome's is in gnome-tweaks because Gnome hates exposed settings.

I'd support unifying behavior between toolkits and apps to provide users with a single point to set their preference, but I use this feature a hundred times a day. I'd also like it to remain the default; *nix desktops should have their own flavor instead of just copying Mac OS or Windows, and middle-click paste has been a part of that flavor for 40 years.