You don’t have to be actively sick to be allergic to the vaccine - just one reason people may not have it. There do need to be policies in place for those who cannot be vaccinated.
There is no such thing to being unable to get the vaccine due to known allergy. (Anyone May have an unpredicted allergic reaction).
For the rare cases where a person does have a known allergy to 1 of the ingredients in 1 of the vaccines, then get 1 of the other vaccines.
Anybody who isn’t vaccinated should not be taking the risk of high risk public venues. Whether they have an “excuse” or not.