I also tried to find a way to anonymize or to delete user records from the database but found no solution yet on Moodle 3.6.3+ except manually changing the data directly in the database mdl_user table.
Yes I am. When a student finishes his study program, we don't have the rights to keep his personal data in Moodle database anymore except if there is some legal document that is forcing us to keep the data. GDPR prohibits keeping the personal data that is not needed anymore.
Depending on funding rules, you may have to keep data for an agreed period. Check with your funding agency.
You should have a DPO in the organisation who can tell you specifically what you do and don't need to keep.
Of course I already checked with our DPO. We're talking here about the data that we don't have to keep and that we are not allowed to keep according to GDPR.