I haven't got "unlucky" on an international BA flight in 15+ years. So no, it doesn't just depend on the plane.

I flew Cape Town to LHR in January on BA and my entertainment system was stuck in a loop, booting into RedHat over and over again

But at least it's finally the year of Linux in the headrest.

It's not new. I had a 4-hour wait at the terminal to get to a KLM plane in Chengdu a bit over 10 years ago, this was perhaps 2006. The reason became clear when they informed us upon boarding that "the in-flight entertainment system is unfortunately not available, but it is not connected to flight systems and has no impact on flight security".

The entertainment system on the seat backs was Linux in a reboot loop.

Does Turkish Airlines have airplanes in their fleet without seatback screens? Do these planes fly international routes?

Not sure about TA, but my experience is that 95% of this is just about when the planes get bought. Entertainment systems solve the "keep people happy problem" for relatively cheap. But it's pretty expensive to replace all the seats just for that.

JAL/ANA have very new planes with cool entertainment systems, but they also fly their older planes on some routes with a bunch of stuff that looks like it's powered by VCR.

To your question: they probably don't for their major international flights.

I can't think of a single European airline that does not have seat-back IFE on long-haul flights. The same goes for the ME3 airlines.