PUBG and Fortnite come to mind. I know they still both have decent sized player bases, but there was a time where they were the games, and now I don't hear about them very much.
Other people covered these, but a lot of the casual games made by PopCap in the 2000s, namely PVZ, Peggle, and Bejeweled. I still honestly play them to these day, especially Peggle. They're super fun games, I can recommend them to anyone as just chill games. But they all (ESPECIALLY PVZ) were really big until they eventually just disappeared.
Splitgate was pretty big a few years ago because it was a competitive FPS but with portals, and now there's currently 19 active players on Steam... Team Fortress 2 was also fucking MASSIVE, and it still has a playerbase, but it's so old that it's not receiving many updates and it's a shell of its former self. I still play it though...
Arc Raiders also has faded out... it still has a player base, but not even a year ago it was HUGE, and now I haven't hear much talk about it.
Oh, and remember Agar.io and Slither.io? Those were huge in ~2016 or so.
It's funny, there's so many games that have come and gone in such a short time.