This is due to Steam adding a new checkbox that requires the game's developer to specifically click an extra checkbox to signify compatibility with Catalina (and optionally, specify if the binary is notarized and/or signed [don't recall which, or is possibly both]).
Steam almost always already knows if the game has a '64-bit macOS depot' configured -- ie ships 64-bit binary for macOS -- though in some cases the 'depot' may be configured as "macOS - All" in which case Steam may not know for sure.
When Catalina released, they defaulted the checkbox to "No" regardless of the game depot configuration. Any remotely recent game that supports macOS almost certainly is shipping a 64-bit binary (been quite a while since OSX was 32-bit only) -- so odds are very good that games supporting macOS will work with macOS Catalina, at least likely for anything in the last 5 years if not longer.
In our case, it took several weeks to find the checkbox to update our store page. In all fairness, Valve sent out an email alerting of the concern ahead of time; we just didn't read it thoroughly enough to realize that the default setting would be "No" even when explicitly having a 64-bit macOS binary configured.
