I'm not sure if this is the right place to ask, since libsteam_api.so is technically separate from the steam client, but it seems somewhat related to previous feature requests for aarch64 compatibility for the steam client.

It is my understanding that the libsteam_api.so library uses sockets to communicate with the steam client, which means the steam client itself can run under (somewhat slow) x86 emulation, while the libsteam_api.so library and the game that links to it can run natively. This would require much less effort than porting the entire steam client, while not necessarily giving up performance since the game itself will run natively.

aarch64 support was recently added for MacOS, so I assume it should be trivial to add a build target for aarch64 Linux.
The example game from the steamworks SDK compiles fine (even an old version from before the MacOS aarch64 support was added, just needed to mess with the LDFLAGS a bit), so it should be possible to use that for testing. The compiled game launches, but without a real libsteam_api.so it doesn't get very far.

For proper support, presumably steam_runtime for aarch64 should also be added, but since that doesn't depend on any proprietary code it should be possible for any interested party to take care of that (ubuntu already has aarch64 support, which should be quite helpful in that regard)

an aarch64 build of gameoverlayrenderer.so would also be nice, but it is my understanding this is not a hard dependency for any game so it's not a big deal to have it missing