Can you help with this case or am I and others just stuck with the mistake of using them?
As far as I can tell, their use case, in general, isn't covered by the rights to erasure[1]
1. https://ico.org.uk/for-organisations/guide-to-data-protectio...
Can you explain your thinking about why you believe that? I would have interpreted the guidelines in the opposite way.
Telegram does permit individual manual deletion of group conversation messages for all parties, and bulk deletion of 1-1 conversation messages for everyone, just not the group chats. It's incredibly inconsistent even by their own standards.
Reading through the reasons in the "When does the right to erasure apply?" on the link I posted it is not obvious if any of the listed ones apply.
As for the syncing - if they are fully encrypting everything but you have the keys on those devices and they dont have them, what they have is not considered usable by them data. I am not entirely sure how Telegram operate anymore though - if there is no syncing at all, then yes they likely store it in an accessible by them way.