A new answer on Who gets Bitcoin transaction fees? has spawned discussion on the paradigm shift from selection by priority (= coin age × spent value) to selection by fee rate (= fee / transaction size).
It's clear that Bitcoin Core v0.12.0 has turned selection by priority off by default, but I am wondering whether it follows that all miners have discontinued using it.
Is it known whether any miners are still selecting the transactions to go into their blocks using selection by priority?
Generally, no miners still use the priority system for selecting transactions. You can easily test this by spending a very old UTXO (e.g. 1 BTC that has remained unspent for several years) and paying no transaction fee. If miners still are using priority, your transaction should be confirmed relatively quickly (hint: it won't be). Otherwise, your transaction will not confirm for several weeks.
The answer of this question depends on the setup of the miner who succeeded to solve the block. In general the priority still matters most; only the spare additional resources up to the maximum block size include transactions not paying the recommended amount of network fee(s).
As a miner you can even setup a minimum (or maximum) transaction fee amount needed to get your transaction included into a new block:
At all times, Bitcoin Core will cap fees at
-maxtxfee=<x>(default: 0.10) BTC. Furthermore, Bitcoin Core will never create transactions smaller than the current minimum relay fee. Finally, a user can set the minimum fee rate for all transactions with-mintxfee=<i>, which defaults to 1000 satoshis per kB.
confirmation age × spent value. Priority was replaced by fee rate as the main selection metric. That's exactly what this question is about.