https://doc.rust-lang.org/nightly/rustc/platform-support.html

Tier 1 targets can be thought of as “guaranteed to work”. The Rust project builds official binary releases for each tier 1 target, and automated testing ensures that each tier 1 target builds and passes tests after each change.

https://doc.rust-lang.org/nightly/rustc/target-tier-policy.html#tier-1-target-policy

The target must build and pass tests reliably in CI, for all components that Rust’s CI considers mandatory.

Github is removing support for CI on that target, and since that’s a requirement for Tier 1 support the target has to be downgraded.