It's the end of an era, at least for . On Feb. 15, 2024, Valve will for macOS 10.13 (High Sierra) and 10.14 (Mojave). As Mojave was the last to support 32-bit applications, 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 in October 2021, and 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 , 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."