AFAIK, Y+ is completely gone on IST-NRT from Nov. Asian routes - IST-CAN, IST-PEK, and IST-ICN only.

If you want to book stopovers and/or open jaw, use third party website like

travelocity.com

and go into detailed booking with TK and Premium Economy, this will generate the desired booking combinations.

I used to provide updates on how to book Y+ with shorter segments in Y and stopovers but with TK ending Y+ services it's no longer worth my time. If they change equipments there will be more hassles involved with third party booking sites.