OSM originated in the UK so it uses UK norms for most things. You wouldn't need to tag a motorway as not allowing pedestrians because that's already implied by the fact that it's tagged as a motorway.

What OP was (probably) doing was tagging roads that aren't motorways and that have no sign banning pedestrians with `foot=no`, which is only supposed to be used in situations where walking is illegal or impossible. Eventually more experienced mappers noticed what OP was doing and reverted a bunch of his edits, which isn't a great feeling, particularly if you've been mapping that way for a while before anyone noticed

(If OP was using StreetComplete I'd blame StreetComplete for this more than the OP. They used to have a quest for pedestrian walkability that was poorly worded, which they've since removed because it was leading to exactly this kind of confusion.)