Model Registry stages were deprecated in favor of aliases and tags. What was wrong with stages, and how does the alias workflow look?

Problems with stages: exactly one version per stage per model, so champion/challenger or per-region rollouts needed hacks; the vocabulary was hardcoded, so teams overloaded meanings; and concurrent stage transitions in multi-user setups hit race conditions with no optimistic concurrency. The replacement model: an alias is a named pointer to a specific version — setting @champion to v12 is one atomic pointer update, and rollback is pointing it back to v11 (instant, auditable, no artifact copying). You can have unlimited aliases expressing whatever topology you run (@champion/@challenger, @prod-us/@prod-eu, @shadow). Tags carry the rest (validation status, approver, ticket link). CI/CD integrates cleanly: the deploy job resolves models:/name@champion at rollout, promotion is 'run validation suite -> set alias', and audit comes from registry events. Migration note: stage APIs still exist deprecated; new pipelines should never reference them, and old ones should map Production->@champion, Staging->@challenger (or better, to explicit env aliases).