If those accounts have no activity, they’re harmless. They are invisible to other users (including a public user list). And user profiles, regardless of their trust level, are disallowed in robots.txt and not visible in search engines.
Plus, inactive accounts are periodically cleaned up, see Clean up inactive users after days setting (“Number of days before an inactive user (trust level 0 without any posts) is removed. To disable clean up set to 0.”).
It’s triggered by the CleanUpInactiveUsers SideKiq job.