In short, “tankie” is a thought-terminating phrase thrown into any conversations where people want to get others to terminate all thought coming from people to their left. It gets thrown at communists by other communists, it gets thrown at communists by anarchists, it gets thrown at all of the above by socdems, it gets thrown at all of the above by liberals, and it gets thrown at all of the above including liberals by fascists.
Its function in actual practice is that it moves discourse and spaces rightwards. Anyone using the phrase should be completely disregarded as a person that wants you to keep your brain closed.
EDIT: Downvoters literally downvoting actually published leftist internationalist author saying exactly what I just said here because they don’t want to stop using it as a thought-terminator and don’t consider the fact that all they’re doing is moving things rightwards when they use it.