Perhaps one day the OpenBSD folks will figure out how to completely prevent user programs from making syscalls. It seems they are mostly there but still not quite. Please don't mention WASM in your replies.
Or if you're trying to solve a problem, what are you doing that pledge() doesn't cover? For that matter WASM.... would do that, so why not use it?
As a result, you can disable all syscalls for your program with one simple request: 'exit()'
There are alternative to syscall. For example, writing to shared memory. Shared memory as IPC is hard. And you need some syscall to set the memory up.. .