Incorrect title: ".... in the US" missing. No guarantee, maybe no possibility that such restrictions will be in non-US countries. Perhaps we'll just a whole lot of foreign trips, or (somehow) smuggling nasal or physical vaccines.
Fortunately, COVID doesn't seem as bad now, but if the MMR and other childhood vaccines, flu shots, shingles shots, ... End up with similar restrictions, people might not want that aspect of the 50's and 60's to return. Sterility, deafness, ... lots of possible effects from childhood diseases if they reappear en masse.