This is unused since cluster-mempool, so it seems confusing to keep around. See the commit that forgot to remove it:
$ git show 216e6937290338950215795291dbf0a533e234cf -U99999 | grep newFeeRate
CFeeRate newFeeRate(ws.m_modified_fees, ws.m_vsize);
- if (const auto err_string{PaysMoreThanConflicts(ws.m_iters_conflicting, newFeeRate, hash)}) {