That's a little harsh.

Rust uses Github, but could easily switch to a self-hosted platform if Microsoft became opposed to Rust's goals; (and yes, Microsoft is a Rust sponsor, but not an essential one). Cargo has support for alternate registries built in.

Some of community is on Reddit and Discord, but most technical discussion takes place on Discourse and Zulip. The official "user questions" forum is a Discourse instance. Most subcommunities forming around Rust projects use Zulip instances.

The Rust community uses proprietary services when convenient, but it's hardly dependent on them.