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

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const CBlockIndex* pindex;
    int blockcount = 30 * 24 * 60 * 60 / Params().GetConsensus().nPowTargetSpacing; 
    if (request.params[1].isNull()) {
        LOCK(cs_main);
        pindex = chainman.ActiveChain().Tip();
    } else {
        uint256 hash(ParseHashV(request.params[1], "blockhash"));
        LOCK(cs_main);
        pindex = chainman.m_blockman.LookupBlockIndex(hash);
        if (!pindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
        if (!chainman.ActiveChain().Contains(pindex)) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Block is not in main chain");
        }
    }
    CHECK_NONFATAL(pindex != nullptr);
    if (request.params[0].isNull()) {
        blockcount = std::max(0, std::min(blockcount, pindex->nHeight - 1));
    } else {
        blockcount = request.params[0].get_int();
        if (blockcount < 0 || (blockcount > 0 && blockcount >= pindex->nHeight)) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid block count: should be between 0 and the block's height - 1");
        }
    }
    const CBlockIndex* pindexPast = pindex->GetAncestor(pindex->nHeight - blockcount);
    int nTimeDiff = pindex->GetMedianTimePast() - pindexPast->GetMedianTimePast();
    int nTxDiff = pindex->nChainTx - pindexPast->nChainTx;
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("time", (int64_t)pindex->nTime);