> We already had a universal copy/paste in Linux: Mark with left mouse button, paste with middle button.

We still have it, at least for text. I just copied the little quote above with left mouse highlight, middle mouse paste while writing this comment in Firefox in Gnome 48 with Wayland. I use it a lot on the terminal.

What doesn't appear to work anymore (at least in Wayland) is copying a selection as a bitmap into a bitmap editor, ie. paint. Only a few decades ago that used to work, I remember that.