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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CCoinsStats stats{hash_type};
    stats.index_requested = request.params[2].isNull() || request.params[2].get_bool();
    NodeContext& node = EnsureAnyNodeContext(request.context);
    ChainstateManager& chainman = EnsureChainman(node);
    CChainState& active_chainstate = chainman.ActiveChainstate();
    active_chainstate.ForceFlushStateToDisk();
    CCoinsView* coins_view;
    BlockManager* blockman;
    {
        LOCK(::cs_main);
        coins_view = &active_chainstate.CoinsDB();
        blockman = &active_chainstate.m_blockman;
        pindex = blockman->LookupBlockIndex(coins_view->GetBestBlock());
    }
    if (!request.params[1].isNull()) {
        if (!g_coin_stats_index) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Querying specific block heights requires coinstatsindex");
        }
        if (stats.m_hash_type == CoinStatsHashType::HASH_SERIALIZED) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "hash_serialized_2 hash type cannot be queried for a specific block");
        }
        pindex = ParseHashOrHeight(request.params[1], chainman);
    }
    if (stats.index_requested && g_coin_stats_index) {
        if (!g_coin_stats_index->BlockUntilSyncedToCurrentChain()) {
            const IndexSummary summary{g_coin_stats_index->GetSummary()};
            
            
            if (pindex->nHeight > summary.best_block_height) {
                throw JSONRPCError(RPC_INTERNAL_ERROR, strprintf("Unable to get data because coinstatsindex is still syncing. Current height: %d", summary.best_block_height));