Related to Trouble with federated calendar shares (Is still an issue.)
see also See calendar from another nextcloud instance with authentication
Now I found this: Share Calendar over Federated-Cloud-ID where it says, it was scheduled for Nextcloud 13.
If I follow the provided link to the github issue (Federated calendar and contacts sharing [$360.00] · Issue #1440 · nextcloud/server · GitHub) I find it still open and with a bounty of 360$.
What would be needed to make it finally happening?