Is it possible you might be looking at this old support page from when MacOS first dropped support for 32 bit apps but Valve was still supporting the 32-bit client for older MacOS versions? The current page is this one. I doubt they'll be removing games from your libraries at any point, but the new article makes it clear that they aren't supporting the 32-bit Steam client or 32-bit games on Mac anymore and make no guarantees that they'll continue to work when the client stops getting updates.

After February 15th, 2024, we will no longer support macOS 10.14 or earlier and we are unable to guarantee continued functionality of 32-bit macOS games after that date.