I must correct myself. We had no diplomatic relations, no embassy or consulates, and had no transportation into North Korea when Otto went, but not a prohibition from the US side on citizens going there:
“The Swedish Embassy in Pyongyang operates as a conduit between the U.S. and North Korea, which do not maintain diplomatic relations.
Up to now, the U.S. policy has been that it “strongly recommends against all travel by U.S. citizens to North Korea.””
There were hard core left wing (Stalinist oriented! - Yes! they exist!) US groups who sponsored “highly controlled” tours into North Korea. I don’t know if it’s still going on.