I really don't understand why people, including mods, are so trigger happy to close questions with the reason:
Questions seeking advice on company-specific regulations, agreements, or policies should be directed to your manager or HR department. Questions that address only a specific company or position are of limited use to future visitors. Questions seeking legal advice should be directed to legal professionals.
This question is CLEARLY NOT asking about company-specific regulations, agreements, policies, or asking about legal advice.
The content of the question is (emphasis mine):
Is it proper etiquette to inform your work colleagues that you are getting a covid booster shot in the future and you may be put of commission on that day?
Or should you keep it to yourself?
So the question has nothing to do with workplace rules. In fact, the OP has carefully ensured that it shouldn't be interpreted that way. They explicitly said "etiquette".
An answer may suggest: "You should keep in mind that there may be workplace policies that require you to disclose." The potential existence of a policy covering such a thing doesn't mean we should close the question.