Are seems correct to me. You wouldn't say burgers is back on the menu.
Yeah, While I can't say I've ever heard someone say carnita, carnitas is plural.
It's a plural noun, breh.
Well of course. If they were made with anything else, they wouldn't be carnitas.Either way, Carnitas is delicious. PORK, of course.
Which establishment is offering them?
Del Taco, 2 Burritos for $5. Google beef Carnitas, it also exists.
Carnitas are marinaded (usually) and grilled chunks of pork. Del Sol's version looks shredded but I suppose that's for the best as it would be more certain that they are cooked thoroughly.
http://www.dictionary.com/browse/carnitas?s=tCarnitas is a food item. It was gone. Now, it (carnitas) is back.
It's not plural in the same sense that soup isn't plural.
http://www.dictionary.com/browse/carnitas?s=t
Plural noun. Soup isn't plural, because the plural of soup is soups.
http://www.dictionary.com/browse/soup?s=t
https://www.merriam-webster.com/dictionary/soup
Notice how it is described as just a noun. Example sentence from second link: For lunch, Olivia's offers a selection of soups, sandwiches, salads, pizza, hotdogs and a meat-and-two soul plate.
Sure, though I think you're missing the point.
The word Carnitas is also used to describe a singular substance with a customary method of preparation, like stew describes little meats and pieces of carrot, that has a somewhat different meaning than the specific literal definition of the word and thus can properly be used in that manner. See Chipotle's usage that I posted above.

 
				
		