Let’s be generous in our definition of “transgressive” - if a food could shock you and/or people whose opinions you care about, for cultural, religious, health, aesthetic, or other reasons, it counts.
I ate endangered sea turtle in Micronesia in 1988. The turtle was a gift, given with great respect, from some outer islanders to the president of the Federated States of Micronesia. The roasting and consumption was done with considerable fanfare. It was one of those situations where there was no morally unambiguous answer, it was simply a question of which principle one decided to violate.
One time I got a red wine flight at Epcot, drank a little out of each, then, at the counter, in front of the French-person-from-france person who was manning the counter, dumped them into one glass for more convenient carrying (they also balanced each other out IMO but the counter person was pretty shocked.)
I’ve had shark fin soup (at a relatively fancy place in China, so I presume it was real shark fin). I’ve also had balut (boiled egg with a duck embryo inside). I don’t think I’ll have either of those again.
I’ve had veal, foie gras, rabbit and horse. Nothing unusual about the first three, I suppose. I’ve given up the first two for ethical reasons. Still love me some hasenpfeffer. The horse was a long time ago and it’s unlikely to be on the menu again. ETA: Sheep and goat too, if memory serves.
Had a taste of my friend’s foie gras when we were in Paris. Was interesting. My dad makes duck liver pate from results of his friend’s duck hunting, which I really enjoy.
I’ve eaten veal sausage (weisswurst/kalbsbratwurst) and will probably eat it again. Veal is not demonized here in Switzerland, except by those who are against eating any meat. I don’t buy it to cook at home, just eat it at a restaurant.
I’ve tasted rabbit, which I didn’t like. Both rabbit and horse are acceptable meats here.
I rather enjoy telling people I’ve eaten rat. (And I tell them it didn’t taste like chicken, it tasted very much like rat.)
When I worked for the New Zealand Wildlife service, some university people were doing work on the Polynesian Rat on the island where I worked and were trapping them. Since the rats were a favorite dish of the Maori, one day they decided to fricassee a few and try them out. A reporter who was sharing the bunkhouse with us was absolutely horrified.
They weren’t very good. The meat was very dark. As rodents go, I much prefer guinea pig (which I’ve had several times in Peru) and Paca (a large jungle rodent). Muskrat and squirrel aren’t that great. I quite like rabbit (not a true rodent, though). Other “shocking” foods I’ve had are alligator, caiman, iguana, sea turtle, kangaroo, whale (the last three were canned, over 50 years ago, as a gift I got as a joke), and large beetle grubs on a stick.
(though they probably weren’t)