`banlist.dat` was introduced in https://github.com/bitcoin/bitcoin/pull/6310, storing nodes that were automatically banned (due to exceeding the misbehavior threshold) and manually banned (through the `setban` RPC).

Automatic bans were removed in https://github.com/bitcoin/bitcoin/pull/19219 and replaced with a discouragement filter. That filter is not saved to disk/persisted over shutdown/startup. The `banlist.dat` therefore now only contains addresses that have been banned through the `setban` rpc.

Since https://github.com/bitcoin/bitcoin/pull/15935, we have a specific file for configuration that is updated through the RPC and persisted over shutdown/startup, namely `settings.json`. We should therefore move the manual ban configuration from `banlist.dat` to `settings.json`. Doing so has a couple of benefits:

- `settings.json` is human readable and easily analyzable by any json parser.
- we wouldn't need to maintain custom [de]serialization code for the `banlist.dat` file.

`banlist.dat` is expected to be fairly small and infrequently updated, so disk space/performance are not huge concerns.
fs::path pathTmp = gArgs.GetDataDirNet() / tmpfn;
    FILE *file = fsbridge::fopen(pathTmp, "wb");
    CAutoFile fileout(file, SER_DISK, version);
    if (fileout.IsNull()) {
        fileout.fclose();
        remove(pathTmp);
        return error("%s: Failed to open file %s", __func__, pathTmp.string());
    }
    
    if (!SerializeDB(fileout, data)) {
        fileout.fclose();
        remove(pathTmp);
        return false;
    }
    if (!FileCommit(fileout.Get())) {
        fileout.fclose();
        remove(pathTmp);
        return error("%s: Failed to flush file %s", __func__, pathTmp.string());
    }
    fileout.fclose();
    
    if (!RenameOver(pathTmp, path)) {
        remove(pathTmp);
        return error("%s: Rename-into-place failed", __func__);
    }
    return true;
}
template <typename Stream, typename Data>
bool DeserializeDB(Stream& stream, Data& data, bool fCheckSum = true)