`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.
void Ban(const CSubNet& sub_net, int64_t ban_time_offset = 0, bool since_unix_epoch = false);
    void Discourage(const CNetAddr& net_addr);
    void ClearBanned();
    
    bool IsBanned(const CNetAddr& net_addr);
    
    bool IsBanned(const CSubNet& sub_net);
    
    bool IsDiscouraged(const CNetAddr& net_addr);
    bool Unban(const CNetAddr& net_addr);
    bool Unban(const CSubNet& sub_net);
    void GetBanned(banmap_t& banmap);
    void DumpBanlist();
private:
    void SetBanned(const banmap_t& banmap);
    bool BannedSetIsDirty();
    
    void SetBannedSetDirty(bool dirty = true);
    
    void SweepBanned();
    RecursiveMutex m_cs_banned;
    banmap_t m_banned GUARDED_BY(m_cs_banned);
    bool m_is_dirty GUARDED_BY(m_cs_banned);
    CClientUIInterface* m_client_interface = nullptr;
    CBanDB m_ban_db;
    const int64_t m_default_ban_time;
    CRollingBloomFilter m_discouraged GUARDED_BY(m_cs_banned) {50000, 0.000001};
};
#endif