Might be classified as a bug, might be classified as a feature, I don’t know. I have chosen to classify it as a bug because the system doesn’t behave as expected 
If you add a custom user field dropdown and make it required upon signup, people are able to just leave the field at the first option, and continue (so 50% of the users seem to be from Afghanistan for example)
So in effect, they’re not actually required.
It would be better if people were forced to actually choose a value from the dropdown. I think this can be accomplished the best by inserting a “-- please choose --” entry at the top.