Steam has kept support for macOS Mojave and macOS High Sierra alive so far to keep 32-bit games running on older Macs, but that support is ending on February 15.
Steam
Steam is a multi-platform game storefront and distributor available on Mac and PC. It hosts thousands of games and is the most popular place to get video games on computers.
Macs may not be known for gaming, but it's always been possible to run many titles, even from Steam. However, Steam is ending support for older versions of macOS as of February 15, 2024.
From Steam's statement:
As of February 15th, 2024, Steam will officially stop supporting the macOS 10.13 (High Sierra) and 10.14 (Mojave) operating systems. After that date, existing Steam Client installations on these operating systems will no longer receive updates of any kind including security updates. Steam Support will be unable to offer users technical support for issues related to the old operating systems, and Steam will be unable to guarantee continued functionality of Steam on the unsupported operating system versions.
Apple stopped supporting macOS High Sierra in 2020 and macOS Mojave in 2021. Steam continued to offer basic updates and support for its client on these operating systems since it was the only way to play 32-bit games.
After the cutoff date, Steam won't be able to guarantee the functionality of games on older systems. Steam reports that 98% of users are already running macOS Catalina or later, so very few users will be affected.
Users stuck on these older operating systems are urged to update to the latest macOS available if possible. Those older operating systems are vulnerable to exploits that have been patched in newer updates.
Steam will cease updating its client for macOS High Sierra and macOS Mojave on February 15, 2024.