Starting February 15, 2024, Steam will no longer provide support for macOS versions 10.13 (High Sierra) and 10.14 (Mojave). The latter is the final version that supports 32-bit games, so such beloved titles as Civilization IV, Half-Life 2, and Metro 2033 will become unplayable on Macs with those versions of macOS.
The decision to discontinue support for these older macOS versions stems from several factors. First up, macOS Mojave is the final version that supports 32-bit games. Apple's move to stop supporting 32-bit applications in macOS 10.15 has rendered many older games incompatible with newer macOS versions. This change will also be reflected in the Steam store, which will no longer list games with only 32-bit macOS binaries as compatible with Mac at the end of 2023.
Valve says that over 98% of Mac Steam users have migrated to newer macOS versions, so very few people will likely be impacted. Meanwhile, users of the older versions are urged to upgrade for continued access to Steam features and security updates. Furthermore, while Steam and its games may continue to function on these older versions for a time, their performance and compatibility cannot be guaranteed beyond the February 2024 deadline.
For users still on macOS 10.13 or 10.14, upgrading to a more recent macOS version is essential to get the latest security fixes and new features, but there are Macs that do not support Mojave. If people running those versions update Steam (which is done automatically), they might lose access to 32-bit games. There are likely older Macs are kept specifically by their owners to run legacy applications, including older games.
The technical aspects of Steam's operation are also a factor in this shift. The platform relies on an embedded version of Google Chrome, which does not work with macOS versions older than 10.15. (We saw this happen earlier this year for users of macOS 10.11 El Capitan and 10.12 Sierra.) Future versions of Steam will need macOS features and security updates only present in newer versions, further necessitating the move away from older platforms
Finally, the security implications for users on outdated macOS versions are significant. Apple stopped providing security updates and technical support for macOS 10.13 in December 2020 and macOS 10.14 in October 2021.
