To be clear, tier 2 targets are still expected to be well supported. It just doesn't require CI to pass after every PR (meaning PRs aren't blocked on fixing tier 2 specific issues). However, the target is officially distributed and target maintainers are still expected to fix any blocking issues before a release. If they can't then it's likely the target will be demoted to tier 3.

Windows on ARM couldn't be tier 1 until recently as there weren't Windows ARM github runners. Now that there are I think it likely that it'll be promoted to tier 1.