Tier 1 is the summit of summits - the summum bonum of languages, the highest order to which a language can aspire. Very few ever attain it. Non multa, sed multum: not many, but only those of extraordinary quality. Most languages remain forever in Tier 3, never passing beyond its gates. Of these, scarcely 1% ascend to Tier 2. And from that already distinguished company, a mere 0.1% possess the refinement, depth, and excellence required to cross the final threshold into Tier 1.

Consider what that means: Tier 1 represents roughly the top 0.001% of languages. Pauci sed electi - few, but chosen. The crème de la crème. The aristocracy of languages. Primus inter pares, yet standing at the very edge of what programming language greatness can be.

Ad astra per aspera. Through hardship, to the stars. Tier 1 is not merely another rank: it is the ultima Thule, the farthest frontier, the crown, the apotheosis.

(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.

It means it's about as good as C++ ;)

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It means a language for the very elite special force unit. In MSFT, that means the Ads teams will be able to serve you Ads in excel very quickly. /s