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 CChainParams& chainparams = Params();
    
    
    if (!fs::is_directory(GetBlocksDir(false))) {
        return InitError(strprintf(_("Specified blocks directory \"%s\" does not exist."), gArgs.GetArg("-blocksdir", "").c_str()));
    }
    
    if (gArgs.GetArg("-prune", 0)) {
        if (gArgs.GetBoolArg("-txindex", DEFAULT_TXINDEX))
            return InitError(_("Prune mode is incompatible with -txindex."));
    }
    
    size_t nUserBind = gArgs.GetArgs("-bind").size() + gArgs.GetArgs("-whitebind").size();
    if (nUserBind != 0 && !gArgs.GetBoolArg("-listen", DEFAULT_LISTEN)) {
        return InitError("Cannot set -bind or -whitebind together with -listen=0");
    }
    
    int nBind = std::max(nUserBind, size_t(1));
    nUserMaxConnections = gArgs.GetArg("-maxconnections", DEFAULT_MAX_PEER_CONNECTIONS);
    nMaxConnections = std::max(nUserMaxConnections, 0);