It's the end of an era, at least for Mac gamers. On Feb. 15, 2024, Valve will end support for macOS 10.13 (High Sierra) and 10.14 (Mojave). As Mojave was the last to support 32-bit applications, Steam won't support 32-bit games on Mac anymore.
All things considered, Valve can be commended for supporting users on older versions of macOS for so long. Apple ended support for macOS Mojave in October 2021, and High Sierra support ended in December 2020.
It's possible Valve might have continued to support the two aging operating systems, but it couldn't anymore. As Valve notes, Steam relies on Google Chrome for portions of its UI, and Google already ended Chrome support for macOS High Sierra and Mojave. There isn't much incentive to continue support either; according to Valve, 98% of Steam users are already on newer macOS versions and thus will continue to work as usual.
Those macOS versions don't support 32-bit applications either, so effectively, the latest change already applied to them. With Steam dropping support too, it says it will no longer consider games that only offer 32-bit binaries as "macOS compatible."
This change doesn't mean Steam will immediately stop working on older macOS versions. But Valve won't provide security updates or troubleshooting reports if anything doesn't work right. So upgrading makes the most sense from both a compatibility and a security perspective.
If you previously purchased 32-bit games, you can continue to try to use them. But there's no guarantee something won't break down the road, and then you're out of luck. Unfortunately, if a game is older and isn't selling much anymore, there's not much incentive to provide updated binaries. Eventually, older games will stop working, but that's been a fact of gaming for as long as gaming has existed.