No, you need to install them as well. It’ll work as long as it’s installed, but after they’re delisted, I don’t think steam can even distribute them.
I know I have at least one game “grid: race driver” that was delisted from steam, at some point I must have uninstalled it, and now I can’t download it, it doesn’t even show up in my library. I’ve been trying like hell to find a way to play it, but even pirated versions are being difficult.
Edit: judging from comments and downvotes, apparently this is rare? But it does definitely happen, I’m not making this example up. And I don’t know how you could predict whether a game will just be unavailable for purchase or totally disappear…
Well that’s certainly good news! And I admit I wasn’t aware of that announcement.