Steam will officially stop supporting macOS versions 10.11 (“El Capitan”) and 10.12 (“Sierra”) on September 1, 2023, Valve has announced. The Steam Client will no longer run on those versions of macOS after that date, and those who wish to continue running Steam and any games or other products purchased through the platform will need to update to a newer version of macOS. The latest release of macOS is macOS Ventura, which Apple launched on October 24, 2022, featuring passkeys for Safari, easier ways for organizing workspaces, and new mail improvements.
From a Steam Support article:
This change is required as core features in Steam rely on an embedded version of Google Chrome, which no longer functions on older versions of macOS. In addition, future versions of Steam will require macOS feature and security updates only present in macOS 10.13 and above.
Although support won’t end until the above date, we strongly encourage all macOS 10.11 and 10.12 users to update sooner rather than later. Apple ended security updates and technical support for macOS 10.11 in 2018 and for macOS 10.12 in 2019. Computers running these operating systems, when connected to the internet, are susceptible to new malware and other exploits which will not be patched. That malware can cause your computer, Steam and games to perform poorly or crash. That malware can also be used to steal the credentials for your Steam account or other services.