This article almost makes it as if they’re removing stuff from your library
But it doesn’t remotely imply that? Here are the words/phrases it uses to describe what will happen:
Steam will halt support for macOS 10.13 and 10.14
existing Steam Client installations on those operating systems will no longer receive updates of any kind, including security updates
some games will effectively stop functioning on macOS
The Steam store will stop considering games that offer only 32-bit macOS binaries to be Mac compatible at the end of 2023.
At no point does it say or imply that anything will be removed from your library. In fact, it explicitly says how you can ensure that those games you own will remain playable:
In order to ensure continued operation of Steam and new 64-bit games purchased through Steam, users on these older versions should update to a more recent version of macOS.
Why would they include that if they’re trying to tell people the games will be removed from their library?