Interesting, I am not able to play HL2 on Steam because macOS no longer has 32-bit support and Valve never compiled if for 64-bit but here we are, it’s playable on the same OS in the browser. BTW IIRC there was some method to convert the 32-bit game binaries to make them run on recent macs. I remember doing it.
On paper qemu should be able to do this. The hard part is hardware acceleration for the GPU. Without Apple putting effort into supporting this with e.g. documentation, that's a bit hard. That's also holding back linux support on Apple hardware. But it's a fixable problem that will only get easier as hw gets better and faster over time.
Is it, though?
How Hard Can It Possibly Be to just do a software GL renderer that emulates a mid-2000s Radeon, these days?