Quote Originally Posted by Raymott View Post
"Thank you" is an expressive illocutionary speech act. It has the perlocutionary effect of letting the hearer know that the speaker is grateful.
Speech act - Wikipedia, the free encyclopedia

It's declarative, but it's also a performative act. If you want to explain it in the model of "Declarative, Interrogative, Emphatic, Exclamation" then it's not the latter three, hence it's the first. But it can be described in other ways, depending on whose theory of speech acts you want to use.
Great. I have been learning about J.L. Austin's speech act theory and other theorists' rebuttals recently.