New fun with @SlackHQ...create a private channel, invite a bot and another person. Leave the channel. The other person can't leave. They have to invite someone else before they can leave.
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-_- But this has been an issue for over 2 years, hence the original thread from January 2018
A channel can't be left with zero members. If you don't have permission to archive channels then you're stuck. This is a bug and we'll try to fix it! 
Apologies if we weren't clear — this behaviour is still something we're figuring out the best way to address, so we appreciate your suggestion on improving the experience.