The documentation for nightbot does not have !permit anywhere. There may be other unlisted commands too that work but never made it into the documentation.
Pages I looked for any mention of the command:
https://docs.nightbot.tv/
https://docs.nightbot.tv/spam-protection/links
https://docs.nightbot.tv/commands/regulars
https://beta.nightbot.tv/spam_protection
https://beta.nightbot.tv/commands/default