`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.
LogPrintf("%s\n", e.what());
                strLoadError = _("Error opening block database");
                failed_verification = true;
                break;
            }
            if (!failed_verification) {
                fLoaded = true;
                LogPrintf(" block index %15dms\n", GetTimeMillis() - load_block_index_start_time);
            }
        } while(false);
        if (!fLoaded && !ShutdownRequested()) {
             f
            if (!fReset) {
                bool fRet = uiInterface.ThreadSafeQuestion(
                    strLoadError + Untranslated(".\n\n") + _("Do you want to rebuild the block database now?"),
                    strLoadError.original + ".\nPlease restart with -reindex or -reindex-chainstate to recover.",
                    "", CClientUIInterface::MSG_ERROR | CClientUIInterface::BTN_ABORT);
                if (fRet) {
                    fReindex = true;
                    AbortShutdown();
                } else {
                    LogPrintf("Aborted block database rebuild. Exiting.\n");
                    return false;
                }
            } else {
                return InitError(strLoadError);
            }
        }
    }
     A