Federated sharing will break the encapsulation for the address books: #19575 (comment)

Questions:

• GDPR compliance - Results from call with @ChristophWurst - don't show any federated contacts, only the contacts with the CLOUD property that matches my instance.
• How memory intensive is filtering for the CLOUD property mentioned - handle the filtering via DB query
• Should admins be able to allow / disallow this behaviour - no, this should be disabled by default - i. e. encapsulate system address book no matter if the instance is fedrated or not.
• Are guest users added to the system address book? Yes, they do. Does that need to be avoided?
• Do they have an address book home / user principal, i. e. are they able to access the system address book? Yes they do?

The big question is if guests should be added to the system? @schiessle and I say no, especially since it can be a data leak point in conjunction with federated sharing

Results from testing system address books with federated sharing: each federated share has their own system address book!