std may not yet provide a stable interface for custom allocators (the Allocator API appears to be available in unstable presently), but use of custom allocators is common in no-std rust environments like embedded. Reading through https://github.com/irbull/custom_allocators the Allocator API doesn't seem particularly complicated. I think it's fair to expect that it will stabilize in time.

> That's like saying Rust has GC because GC libraries/runtimes can be implemented in/for Rust.

Quite a few have already been implemented and are available as libraries exactly as you describe, today. I wouldn't phrase that as "Rust has GC" because that might imply that the GC is required. But if your application might benefit from GC, it's certainly available in the language. I might say "GC is optionally available in Rust if desired" to be more accurate.

No, it isn't. Because that distinction is significant if you are using the language in an environment where those libraries are not available or suitable, such as the Linux project which uses a custom fork of Alloc which provides collections for different allocators.