This is about GlobalAlloc, the trait that actually exists in stable Rust. Only static variables can be marked as the #[global_allocator], and statics are always required to be Send + Sync. Of course, if you're really sure that only a single thread will ever exist, you can pinky-promise it by putting a couple unsafe impls on the allocator type. But then the language won't protect you if you do try to allocate or deallocate from multiple threads.

(More practically, you can just write a GlobalAlloc that forwards to an instance of a thread-local allocator. Though that runs the risk of thread-local deinitialization running in an unexpected order.)