Final update: Turkish Airlines officially dropped their mask mandate as of May 30th 2022.
Update: took two flights this week. Zero mask enforcement whatsoever on board. YMMV but it seems like Turkish Airlines is not taking their mask rule seriously.
As per FlyerTalk, masks are still required on paper but enforcement is hit or miss in practice, especially in the last few weeks:
I noticed a big reduction in mask usage at IST from late April (the majority) to earlier this week (less than half). Also on the outbound flights there were announcements about masks, none on the return, with pax being asked to put them on during boarding outbound, not so on way back.
My experience over 8 J class flights in last few months is that on B737 (and presumably other open cabins) mask are required but that crew are fairly relaxed about passengers not keeping them on with drink in hand. On B787 with suites, crew don't bother enforcing masks at all.
Flew CDG-IST-SIN-IST-CDG very recently & didn't see crew enforcing masks aside from the automated announcements.
Flew JFK-IST-ACC yesterday. The crew were quite strict on the JFK-IST leg, however no one seemed to care on the IST-ACC leg.
However masks are not required in Istanbul Airport, so you won't need one for the layover.