Unfortunately, that one doesn't have 64-bit support (requires a full rewrite to get off of carbon). So no Catalina, but it is very impressive it works on Tiger through Mojave with no hitches! That's 10 versions of the OS.
Just as an interesting technical note, MacDraw Retro is another one that supports Tiger through Mojave.
And of course, there's a lot of small programs that support the range as well, sometimes just by chance (a Tiger-era program just happens to work), but you're right it's rare for bigger, more-complex stuff.
