(Link worked for me ...)
This “Tier-1 language” engineering status for Rust means giving internal teams a paved path from local development to production: secure toolchain builds, productive developer tooling, quality workflows, deep platform integration, and compliance with the SDL requirements Microsoft software must meet.
Specifically, it joins a list of existing Tier 1 languages (C++, C#, and TypeScript), as "one of the best-supported languages for internal development at Microsoft"