With the addition of a new Frozen Tier in Elasticsearch, and recent improvements in heap utilization, “frozen indices” (_freeze / _unfreeze endpoints) provide little end-user benefit.

Elasticsearch should deprecate, and then remove, _freeze and _unfreeze endpoints. This work should occur in a way that minimally impacts users, with a large amount of notice, and escape hatches along the way.

7.x:

• Deprecate freeze index endpoint (Deprecate freeze index API #72618)
• Deprecate the ILM freeze action (Adding deprecation logging and deprecation info API for ILM freeze ation #77969)
  - [ ] Warn on startup if frozen indices exist
• Add deprecation issue to deprecation info api that checks whether frozen indices exist in a cluster. (Add frozen index deprecation check. #78915)
• Warn when searching frozen indices? (Warn when searching a frozen index. #78184)
• Warn on use of ignore_throttled parameter? (Deprecate ignore_throttled parameter #77479)

8.0:

• Remove _freeze endpoint (no new frozen indices can be created) (Remove endpoint for freezing indices #78918)
• Make the ILM freeze action a no-op in 8.0 (Make the ILM freeze action a no-op #77158)
• Continue to support the _unfreeze endpoint throughout 8.x (give users an escape hatch for unfreezing existing frozen indices in their cluster)

9.0:

• Remove the _unfreeze endpoint in 9.0
• Automatically _unfreeze any frozen indices upgrading to 9.0
• Continue to retain the lazy engine internally for shared_cache searchable, for the Frozen Tier
• Add migration assistant support for inspecting for frozen indices and freeze actions in ILM policies

CC @jasontedor @ywelsch @dakrone