I'm betting it's their recent CS2 rollout, where instead of adding it as a new listing they released it as an update to CS:Go. This meant the next time you logged in, it downloaded the new game and removed the old. The goal was to funnel people over to the new game.
Upsides: CS2 should immediately have the full player base.
Downsides: CS:Go is no longer available. CS2 is not a drop-in replacement, it has higher system requirements and no longer supports Mac. No opportunity for CS:Go to live on through custom servers.
It's definately an odd situation, but it's very much not the norm. Steam tends to be ridiculously good at leaving your library intact. I still play Rocket League through steam even though it's been Epic exclusive (and free) for years.
That said, Valve isn't running any servers for the game anymore, so matchmaking will no longer work. But you can relatively easily download the game and play on community/private servers.
A recent possibility could be Valve releasing Counter Strike 2 by replacing Counter Strike: Global Offensive entirely. Technically you can still play CS:GO by selecting the version in the Betas drop down (for now at least) but there are plenty of people who purchased CS:GO years ago before it was free and are upset because their "paid" game was replaced by a "free" game.
Paid game replaced by a free game replaced by a different game entirely. That’s an anti-trust lawsuit waiting to happen. At the very least consumer protection law.
I believe Overwatch was originally a paid game, went free-to-play, and then was "upgraded" to Overwatch 2 in a similar way to CS:GO>CS2 did, but I don't remember there being as much anger over it.
