Catalina removed 32-bit support, so Mojave was the last version with it, so Valve has had since October 2019 to get their 64-bit shit together — arguably since June 2018 when Apple informed everybody that Mojave would be the last OS with 32-bit support.
Apple started telling people that 32-bit was deprecated something like fifteen years before it was actually removed. Everyone except video game programmers got the message, which says something about video game programmers.
