For quite some time you could do this manually without much trouble, but you did need to know a few obscure steps to do it right. Basically a matter of finding the ID of the game so you can copy a certain metadata file along with the actual game data.

I used the trick to copy some games to my Steam Deck back in the Fall. I speculated then that they might make this a genuine feature since it's so simple. I'm glad to see it happened.