I'm not in favor of this, mainly because I think having another runtime (WASM) is overkill in this case.
Sandboxing isn't something only achievable through WASM - using Linux as an example you can trivially restrict network access and boom a lot of attack surfaces has gone. In general, we should only do sandboxing when it's low hanging fruit, then leaving the rest to endpoint security software.
Determinism and dependency tracking isn't what you should do with WASM either - you don't have to "automate" dependency tracking so much that you hook into the filesystem APIs. Proc macro developers can do that reasoning on their own.
Finally, about IDEs - I think it might be a fair point, although are there many proc macros that runs in unbounded time? The current issue is with Cargo not optimizing them at all, in reality they shouldn't really cause problems for editors.
Please also note that the proc-macro RPC APIs are fairly high-overhead; they can only run without a performance hit when it's same process, same thread. See https://github.com/rust-lang/rust/issues/56058.