Valve is dropping support for Catalina 10.15 and requires Big Sur 11 for Steam from Feb 15th:
"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 11 and above. Apple ended security updates and technical support for macOS 10.15 in 2022. 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."
help.steampowered.com
"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 11 and above. Apple ended security updates and technical support for macOS 10.15 in 2022. 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."
