Cannot add printer on 6.0 if CUPS configuration persisted from 5.x
I did not try to reproduce this myself yet, but the OP says it happened on 2 different Tails installations where the Printers persistence feature was enabled before upgrading to 6.0: if they try to add a printer, nothing shows up. But this is fixed if they reset the CUPS configuration (Persistent Storage → disable Printers feature → delete data, then re-enable the Printers feature).
This suggests that CUPS config from 5.x breaks stuff on Bookworm.
Tentatively setting as a 6.1 goal because this could potentially impact tons of users, it breaks important use cases, the workaround is not obvious at all, and the fact it was only reported once so far may only be because one does not often add a new printer.