Should I delete profiles for persons we no longer have a legal ground to process personal data?
In general no. Use the separate function Erase personal datainstead. That way all personal data will be erased, but statistics for the number of event participants, alumni registrations, or email recipients will not be affected.
If you alsowant these profiles not to be part of lists, and not count as event participants, alumni registrations, or email recipients, you can also delete their profiles, in addition to erasing their personal data.