`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.
if (!i2psam_arg.empty()) {
        CService addr;
        if (!Lookup(i2psam_arg, addr, 7656, fNameLookup) || !addr.IsValid()) {
            return InitError(strprintf(_("Invalid -i2psam address or hostname: '%s'"), i2psam_arg));
        }
        SetReachable(NET_I2P, true);
        SetProxy(NET_I2P, proxyType{addr});
    } else {
        SetReachable(NET_I2P, false);
    }
    connOptions.m_i2p_accept_incoming = args.GetBoolArg("-i2pacceptincoming", true);
    if (!node.connman->Start(*node.scheduler, connOptions)) {
        return false;
    }
     ***
    SetRPCWarmupFinished();
    uiInterface.InitMessage(_("Done loading").translated);
    for (const auto& client : node.chain_clients) {
        client->start(*node.scheduler);
    }
    BanMan* banman = node.banman.get();
    node.scheduler->scheduleEvery([banman]{
        banman->DumpBanlist();
    }, DUMP_BANS_INTERVAL);
#if HAVE_SYSTEM
    StartupNotify(args);
#endif
    return true;
}