He was last active a year ago but if you trace his wallets, he made some transaction back in the Summer. Since BTC and most likely ETH are going to moon and "might" hit ATH again, we will know if he is still out there whether he moves his ETH coins to an exchange. It would be irresponsible to not take some partial profits at least when ETH hit $1000 again. Since at 2 points in time, it hit a low of $80 and when you see profit that large you should take it.

Typically in most online settings, when someone quits a project, instead of announcing they are retiring from the project, they just stop posting. Pretty much become a ghost and move on to other things. Besides the DAG problem, the software was pretty stable and didn't really need much developement. Maybe he doesn't want to upgrade the software because he wants to give other miner devs some profit sharing. Plus the profits aren't the same as in 2017-2018.