Yes, they are. Typically these are enforced as flexible social norms, though, with individuals free to determine reasonable deviations within convention. They'll rarely be used to remove someone. It's an implied dress code, effectively, but with considerable grey area.

Some weddings are explicit, sure. And that's a couple's choice, to exclude some of their friends and family.

Don't you think there is a difference between accidental and intentional exclusion?