I don't know about this case specifically, but I own Alan Wake on steam which has since been delisted because of music licenses running out. At least for that one, I still own the game on steam and can download, install and play it normally whenever I want, it's just that people cannot buy it anymore through steam. If you're lucky, it's gonna be the same with the adult swim games.

Nothing changes for you, if you already own them. They'll still be in your library like before. Visually, the only difference will be that those specific games will have no store page

I have plenty of since delisted games. Nothing changes, unless they require some sort of always-online component that has also since gone offline.

You can still download and play it. It doesn't get removed from your library.

About the matchmaking, I don't know, sorry.

Pretty sure nothing changes for you.

Obviously in this case, no more updates. If there's a server, it's getting shut down. And no new players will buy it. So player count will decrease until it's a dead game, even if multiplayer is peer to peer. But I doubt that's the case.

I still have Rocket League and might expect the same behavior. It just won’t be sold or appear in search.

You can't see the store page anymore, that's it. I know because I own a delisted game that I don't even remember when/where I got, an astronaut FPS in zero-G asteroids or something. I can still install and try to play, can't actually play because it's multiplayer only and servers are dead.