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.
if (fHavePruned && !fPruneMode) {
                    strLoadError = _("You need to rebuild the database using -reindex to go back to unpruned mode.  This will redownload the entire blockchain");
                    break;
                }
                
                
                
                
                if (!fReindex && !LoadGenesisBlock(chainparams)) {
                    strLoadError = _("Error initializing block database");
                    break;
                }
                
                
                pcoinsdbview.reset(new CCoinsViewDB(nCoinDBCache, false, fReset || fReindexChainState));
                pcoinscatcher.reset(new CCoinsViewErrorCatcher(pcoinsdbview.get()));
                
                
                if (!pcoinsdbview->Upgrade()) {
                    strLoadError = _("Error upgrading chainstate database");
                    break;
                }
                
                if (!ReplayBlocks(chainparams, pcoinsdbview.get())) {
                    strLoadError = _("Unable to replay blocks. You will need to rebuild the database using -reindex-chainstate.");
                    break;
                }