A bit late to the party, but I just tried #12653 and noticed some non-intuitive behavior (on macOS), that's probably worth explaining release notes and help.

Before this new feature I kept the entire datadir on an SSD drive, so I launch with: `bitcoind -datadir=/Volumes/SSD/Bitcoin`.

Initially I naively moved `/Volumes/SSD/Bitcoin/blocks` to `/Users/bitcoin/blocks` and added a line to `/Volumes/SSD/Bitcoin/bitcoin.conf`: `blocksdir=/Users/bitcoin/blocks`.

I then realized that I shouldn't have moved the `index` subdirectory, so I moved that back to `/Volumes/SSD/Bitcoin/blocks/index`. Having an empty `blocks` dir with just and `index` subdir is a bit weird. Perhaps after @jimpo's #11857 is merged, we could just move / assume all indexes to be there (if `-blocksdir` is set)?

Worse, it created a directory `/Users/bitcoin/blocks/blocks`, which seems like a bug. I had to set `blocksdir=/Users/bitcoin` instead.
const int64_t load_block_index_start_time = GetTimeMillis();
            try {
                UnloadBlockIndex();
                pcoinsTip.reset();
                pcoinsdbview.reset();
                pcoinscatcher.reset();
                
                
                pblocktree.reset();
                pblocktree.reset(new CBlockTreeDB(nBlockTreeDBCache, false, fReset));
                if (fReset) {
                    pblocktree->WriteReindexing(true);
                    
                    if (fPruneMode)
                        CleanupBlockRevFiles();
                }
                if (ShutdownRequested()) break;
                
                
                
                
                if (!LoadBlockIndex(chainparams)) {
                    strLoadError = _("Error loading block database");
                    break;
                }
                
                
                if (!mapBlockIndex.empty() && !LookupBlockIndex(chainparams.GetConsensus().hashGenesisBlock)) {
                    return InitError(_("Incorrect or no genesis block found. Wrong datadir for network?"));
                }