Multiple Data Paths (MDP) was deprecated in 7.13.0, and then removed from the master branch several months ago (see #71205). This is a meta issue for restoring MDP for 8.0, to provide a longer deprecation period. The following commits need to be reverted, in order:

• Remove shard multiple data path selections. #75822 2e60f3c (Revert "Removed multiple paths from MetadataStateFormat (#72821)" #78475)
• Removed multiple paths from MetadataStateFormat #72821 76e515a (Revert "Removed multiple paths from MetadataStateFormat (#72821)" #78475)
• Remove multiple paths from ShardPath #72713 08a89c8 (Revert "Remove multiple paths from ShardPath (#72713)" #78485)
• Remove MDP from PersistedClusterStateService #72278 2dfaf7a (Revert "Remove MDP from PersistedClusterStateService (#72278)" #78495)
• Remove multiple paths from elasticsearch-node tool #72351 619a011 (Revert "Remove multiple paths from elasticsearch-node tool (#72351)" #78636)
• Remove multiple paths from NodeEnvironment #72599 b6436c5 (Revert "Remove multiple paths from NodeEnvironment (#72599)" #78655)
• Make NodeEnvironment.resolveIndexFolder singular #72515 7a05d72 (Revert "Make NodeEnvironment.resolveIndexFolder singular (#72515)" #78702)
• Make nodePaths() singular #72514 2dc796a (Revert "Make nodePaths() singular (#72514)" #78801)
• Make NodeEnvironment.nodeDataPaths singular #72432 6a7298e (Revert "Make NodeEnvironment.nodeDataPaths singular (#72432)" #78861)
• Convert index path listeners to single path #72511 fd138f5 (Revert "Convert index path listeners to single path (#72511)" #78935)
• Move the MDP error check into Environment #72498 8602688 (Revert "Move the MDP error check into Environment (#72498)" #78945)
• Make NodeEnvironment.availableShardPaths singular #72441 faede0a (Revert "Make NodeEnvironment.availableShardPaths singular (#72441)" #78985)
• Make NodeEnvironment.indexPaths singular #72438 2a446ad (Revert "Make NodeEnvironment.indexPaths singular (#72438)" #79011)
• Make Environment.dataFiles singular #72327 b1eab79 (Revert "Make Environment.dataFiles singular (#72327)" #79028)
• Convert path.data to String setting instead of List #72282 d933ecd (Revert "Convert path.data to String setting instead of List (#72282)" #79091)
• Fail when using multiple data paths #72184 6aa0735 (Revert "Fail when using multiple data paths (#72184)" #79116)

Also need to apply this fix:

• Fix path.data as comma separated string #76202 (Fix path.data as comma separated string (#76202) #79272)