This sort of idea has been discussed before in this meta: Can we ask about the existence of a game?
The main thrust of the issues outlined in BESW’s answer there is that this sort of question can turn out to be indistinguishable from a recommendation or shopping sort of question. “Recommend me a holiday adventure” and “are there any holiday adventures” have the same answers.
However, as BESW states in their answer, “does X exist” is not, and need not be, wholesale off-topic, and I think putting some reasonable constraints on the question can make it more like a content-identification question. As you have phrased it in the question here, I think you’re pretty close. “Has Wizards of the Coast [or insert publisher] ever published a Thanksgiving or Christmas theme adventure?” is, to me, a reasonably scoped content identification problem. In contrast, “Are there any holiday themed one shots, including 3rd party publications” is just a recommendation question without the word “recommendation” in the title.
Turning now to holiday themed monsters, I think we need to be careful here. Simply asking, “Are there any published holiday themed monsters in 5e?” is likely to invite some speculation about a monster’s connection with some holiday. I can’t rule out the possibility of “vampires are halloween monsters lol” types of answers, and I’d be worried more thoughtful answers would still just be of the kind “hey this thing is related to this cultural tradition, therefore this monster must be too”. This opens the question up to any sort of connection I can contrive, and it turns into the bad sort of list question we want to avoid. However, if you ask the first question, about official holiday adventures, you probably get holiday themed monsters gift wrapped already, if the adventures exist.
