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.

},
    };
}
static RPCHelpMan getchaintxstats()
{
    return RPCHelpMan{"getchaintxstats",
                "\nCompute statistics about the total number and rate of transactions in the chain.\n",
                {
                    {"nblocks", RPCArg::Type::NUM, RPCArg::DefaultHint{"one month"}, "Size of the window in number of blocks"},
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::DefaultHint{"chain tip"}, "The hash of the block that ends the window."},
                },
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {RPCResult::Type::NUM_TIME, "time", "The timestamp for the final block in the window, expressed in " + UNIX_EPOCH_TIME},
                        {RPCResult::Type::NUM, "txcount", "The total number of transactions in the chain up to that point"},
                        {RPCResult::Type::STR_HEX, "window_final_block_hash", "The hash of the final block in the window"},
                        {RPCResult::Type::NUM, "window_final_block_height", "The height of the final block in the window."},
                        {RPCResult::Type::NUM, "window_block_count", "Size of the window in number of blocks"},
                        {RPCResult::Type::NUM, "window_tx_count",  true, "The number of transactions in the window. Only returned if \"window_block_count\" is > 0"},
                        {RPCResult::Type::NUM, "window_interval",  true, "The elapsed time in the window in seconds. Only returned if \"window_block_count\" is > 0"},
                        {RPCResult::Type::NUM, "txrate",  true, "The average rate of transactions per second in the window. Only returned if \"window_interval\" is > 0"},
                    }},
                RPCExamples{
                    HelpExampleCli("getchaintxstats", "")
            + HelpExampleRpc("getchaintxstats", "2016")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    ChainstateManager& chainman = EnsureAnyChainman(request.context);