Is reference counting GC vs. tracing GC a language property or an implementation property? We sometimes hear "Swift doesn't do classic (tracing) GC, it uses ARC."

But I'm not sure if there is anything in the Swift semantics that requires reference counting. It seems that one could build one's own Swift compiler and runtime to use tracing GC.

So what exactly is "reference-counted" about Swift? Apple's implementation or the language itself? Are there parts of the language or the library that so strongly support ARC that we can use that label for the language itself?
Swift guarantees that once the last reference to an object is dropped the object is deinitialized, and the deinit code is immediately run.

Obtaining this kind of guarantee through GC is not possible - at least, not without sacrifying performance. Standard GC mechanisms only ensure the deinit code is eventually run, e.g. at the next GC cycle. For precise semantics, you need a reference count somewhere.