`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.
{
    CSubNet sub_net(net_addr);
    return Unban(sub_net);
}
bool BanMan::Unban(const CSubNet& sub_net)
{
    {
        LOCK(m_cs_banned);
        if (m_banned.erase(sub_net) == 0) return false;
        m_is_dirty = true;
    }
    if (m_client_interface) m_client_interface->BannedListChanged();
    DumpBanlist(); //
    return true;
}
void BanMan::GetBanned(banmap_t& banmap)
{
    LOCK(m_cs_banned);
    // 
    SweepBanned();
    banmap = m_banned; //create a thread safe copy
}
void BanMan::SetBanned(const banmap_t& banmap)