I have no idea how long this has been a thing, and maybe every clipboard works that way, not just Plasma, and I never realized it. It also lets you do things like Rt-click on it and do the regular operations like Search in Firefox. Spaces aren't preserved unless you specifically select them but search engines seem to be able to sort out the words usually.

I post this in case you haven't been part of the 10,000 this day already, like me.

Edit: seems to work in Firefox and LibreOffice, but not Kate or Okular. I'd love to see this as a general feature, it's handy as heck.