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.
break; 
            FILE *file = OpenBlockFile(pos, true);
            if (!file)
                break; 
            LogPrintf("Reindexing block file blk%05u.dat...\n", (unsigned int)nFile);
            LoadExternalBlockFile(chainparams, file, &pos);
            nFile++;
        }
        pblocktree->WriteReindexing(false);
        fReindex = false;
        LogPrintf("Reindexing finished\n");
        
        LoadGenesisBlock(chainparams);
    }
    
    fs::path pathBootstrap = GetDataDir() / "bootstrap.dat";
    if (fs::exists(pathBootstrap)) {
        FILE *file = fsbridge::fopen(pathBootstrap, "rb");
        if (file) {
            fs::path pathBootstrapOld = GetDataDir() / "bootstrap.dat.old";
            LogPrintf("Importing bootstrap.dat...\n");
            LoadExternalBlockFile(chainparams, file);
            RenameOver(pathBootstrap, pathBootstrapOld);
        } else {
            LogPrintf("Warning: Could not open bootstrap file %s\n", pathBootstrap.string());
        }
    }
    
    for (const fs::path& path : vImportFiles) {
        FILE *file = fsbridge::fopen(path, "rb");