My updater offers the chance to update to Nextcloud 23, but I get the following warning:
Is this because these functionalities are now built into the main Nextcloud software (or other apps, e.g. Appointments now seem to be a functionality of Calendar App), or simply because I must wait for the App authors to update the Apps for Nextcloud 23?
…the latter unfortunatly. Also the built in Collabora Server respective Nextcloud Office and various 2FA apps for TOTP or U2F are not ready yet. (but 2FA seems at least to work when you force enable the apps) Nevertheless, I would recommend you to wait with the update until the apps you need are officially supported.
It seems that nextcloud 23 was pushed to stable channel already last night (to 100%?), at least all 3 of my nextclouds received it on stable.
If you have the chance I would definitely wait for at least 2 weeks to give the app authors time to test and update the apps.
Yeah I don’t know why they are pushing this out already when clearly not all “featured” apps are ready yet. I haven’t received a notification yet, but others here on the forum apparently have too.
I usually wait until the first point release on my production instance. And I have a seperate test instance. That way I do not experience any unpleasant surprises when I update my production server, well at least no obvious ones. 
Whoops. My bad. They are live now.
Thanks all for the advice, I will wait until all apps have updates in that case!