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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uint256 filter_header;
    if (!index->LookupFilter(block_index, filter) ||
        !index->LookupFilterHeader(block_index, filter_header)) {
        int err_code;
        std::string errmsg = "Filter not found.";
        if (!block_was_connected) {
            err_code = RPC_INVALID_ADDRESS_OR_KEY;
            errmsg += " Block was not connected to active chain.";
        } else if (!index_ready) {
            err_code = RPC_MISC_ERROR;
            errmsg += " Block filters are still in the process of being indexed.";
        } else {
            err_code = RPC_INTERNAL_ERROR;
            errmsg += " This error is unexpected and indicates index corruption.";
        }
        throw JSONRPCError(err_code, errmsg);
    }
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("filter", HexStr(filter.GetEncodedFilter()));
    ret.pushKV("header", filter_header.GetHex());
    return ret;
},
    };
}
/**
 * Serialize the UTXO set to a file for loading elsewhere.
 *
 * @see SnapshotMetadata
 */
static RPCHelpMan dumptxoutset()