Peer recoveries become faster and use less storage (i.e., no more extra translog) with soft-deletes. Soft-deletes has been enabled by default since 7.0. We should make it mandatory in 8.0, so we can simplify the logic in the engine, translog, recoveries, and other components. This meta issue tracks works to be done on this story.

• Always enable soft-deletes in InternalEngine (Always use soft-deletes in InternalEngine #50415)
• Deprecate indices without soft-deletes (Deprecate indices without soft-deletes #50502)
• Prevent creating new indices without soft-deletes (Make soft-deletes mandatory in 8.0 #51122)
• Uses Lucene index in primary promotion (Use Lucene index in peer recovery and resync #51189)
• Uses Lucene index in peer recoveries (Use Lucene index in peer recovery and resync #51189)
• Remove translog retention policy (Remove translog retention policy #51417)
• Deprecate translog retention settings (Revert "Begin moving date_histogram to offset rounding (backport of #50873) (#50978)" #51239)
• Reject translog retention settings in 8.0 (Remove translog retention settings #51697)

More tests for corner cases

• Shrink/split indices (see Make soft-deletes mandatory in 8.0 #51122)
• Indices without _source (Use Lucene index in peer recovery and resync #51189)
• Frozen indices (see Always use soft-deletes in InternalEngine #50415 (review)) (Account soft-deletes in FrozenEngine #51192)
• Snapshot/restore (see Fix X-Pack Indices Breaking Repository BwC Tests (#51120) #51125)

Cleanup:

• Clean up soft-deletes setting in CCR (Clean soft-deletes setting in ccr tests #51113)
• Clean up soft-deletes setting in tests (let's do it in 9.0 so we can back port to 8.0)