FYI played around a little bit and it turned out J class was available on my routing, however only as married segment, separately the flight from Asia to Istanbul and Istanbul to Warsaw there was J0.

I called the airline and they told me I can't upgrade with miles, however I asked if I can upgrade with a voucher and they told me I can.

I refused, however,
it seems married segment is enough for voucher upgrade because you upgrade the whole route,
with miles we currently upgrade each segment separately, hence I believe there's J required on separate segments.

I switched my dates and managed to find J class on both segments separately and I was able to upgrade the Asia-Istanbul flight via website with two clicks. In general very good experience.

Hope it helps