I can try to give some guidelines, I'd love to see improvements (and also like to know if anyone points out I'm wrong :-)
Legitimate de-platforming is
a) when an individual or organization is removed from the platform based on a request from law enforcement followed by a review by the platforms staff to verify that the request is genuine and that the reasons stated are valid.
The account should still not be deleted for a certain number of days, and if requested by the user or organization, relevant details should be made available to them to take fight it in court against the relevant law enforcement agency.
b) when an individual or organization is removed from the platform based on an written policy, known by the users of the platform in advance and verified by a second team. Upon request an explanation of why the account was deplatformed should be made available.