RPCs that return detailed information about mempool entries (e.g. `getmempoolentry` and `getrawmempool(verbose=True)` include the fields `fee`, `modifiedfee`, `descendantfees`, and `ancestorfees` which were marked as "DEPRECATED" and replaced with a `fees` object field in #12240 (since v0.17).

We should remove these fields, but first, properly deprecate them using `-deprecatedrpc`.

#### Useful skills:

- basic understanding of Bitcoin Core RPC interface

#### Want to work on this issue?

For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md) before opening your pull request.

{
    
    AssertLockNotHeld(cs_main); 
    UniValue result(UniValue::VOBJ);
    result.pushKV("hash", blockindex->GetBlockHash().GetHex());
    const CBlockIndex* pnext;
    int confirmations = ComputeNextBlockAndDepth(tip, blockindex, pnext);
    result.pushKV("confirmations", confirmations);
    result.pushKV("height", blockindex->nHeight);
    result.pushKV("version", blockindex->nVersion);
    result.pushKV("versionHex", strprintf("%08x", blockindex->nVersion));
    result.pushKV("merkleroot", blockindex->hashMerkleRoot.GetHex());
    result.pushKV("time", (int64_t)blockindex->nTime);
    result.pushKV("mediantime", (int64_t)blockindex->GetMedianTimePast());
    result.pushKV("nonce", (uint64_t)blockindex->nNonce);
    result.pushKV("bits", strprintf("%08x", blockindex->nBits));
    result.pushKV("difficulty", GetDifficulty(blockindex));
    result.pushKV("chainwork", blockindex->nChainWork.GetHex());
    result.pushKV("nTx", (uint64_t)blockindex->nTx);
    if (blockindex->pprev)
        result.pushKV("previousblockhash", blockindex->pprev->GetBlockHash().GetHex());
    if (pnext)
        result.pushKV("nextblockhash", pnext->GetBlockHash().GetHex());
    return result;
}
UniValue blockToJSON(const CBlock& block, const CBlockIndex* tip, const CBlockIndex* blockindex, TxVerbosity verbosity)