Well, finally I'm getting my booster shot tomorrow - somewhat late, but it could have been worse.
They tell me it's a Pfizer vaccine (after 2 doses of AZ). Over here, Pfizer's vaccine have been authorized for use up to 90 days after the expiration date (not to throw them away then they expire, perhaps due to mismanagement), and it seems a considerable percentage are indeed expired, though I do not know the actual numbers.
I'm not sure what impact - if any - that would have on effectiveness if I get an expired vaccine. I know many drugs are okay after the expiration date, but I'm not sure for how long that would be for the Pfizer vaccine.