Since Apple dropped 32 bits support with macOS 10.15, Valve has added a property in the Steam database for developers to manually specify if their game is 64 bits (by adding "macos64" in common/osextended), with Steam assuming it's 32 bits by default and showing an alert to Mac users that such games aren't compatible with their OS.

Many developers do not seem to be aware of that fact and there are quite a few 64 bits games for Mac that are wrongly reported as 32 bits. This still happens today (the last example in mind was Braid, Anniversary Edition last month, which was since corrected).

Now that Steam has dropped support for 32 bits versions of macOS altogether, and that 5 years have passed since macOS apps have first had to be 64 bits, it seems reasonable to switch the default to 64 bits for every new game added to the database to address that issue.