I’ve done the migration from slack when mattermost was still version 3.x (maybe it was 3.3 or something around) and these users have been hanging there still.
Are they required for anything? I’d imagine that post ownership was transferred to right users after the posts were imported?
The platform user delete command delete permanently the user and any user related data in mattermost, it includes the posts, channels, webhooks, etcetera. Probably, a better option is to use platform user deactivate which mark the user as deleted in the database, but not delete any data.