I went and stayed with a friend in Thessaloninki last summer, and can confirm that the toilet paper etiquette is still not to flush. She has a beautiful 2 floor apartment, and any blocking of the drains has to be dealt with by the inhabitants acting together, though some don't pay their share.
No flushing the paper in Thessaloniki Airport, either.
The bore of the drains is one of the problems, I imagine.
Anyway, while I was staying there, my friend had to repeatedly remind me "Sequential, don't flush the toilet paper." I learn slowly, and had a few times when I realised what I was about to do, and had to fish out the toilet paper 
When I went to the Greek Islands in the early 1980s, the toilet paper thing was, of course, a thing, but having read "My Family and Other Animals" I had heard of the practice. I met a few young American people: I believe the islanders were in despair, because of course these Americans unthinkingly took a long shower every day as a matter of course...