Fastmail does this

Migadu used to do this till they forgot to renew the signing certs for their profiles, and felt it was too much hassles.

mailbox.org still gives out those profiles... but with no renewal...

Is this a thing on any non-Apple systems too? Is it additive or does it overwrite, on clients you've seen? If you can just go places and add their caldav servers with events, that's a semi viable way to share calendars?

iOS/MacOS, and Linux are great with this.

Mobileconfig is an Apple-only MDM but it works on iOS and MacOS.

There’s webcal:// protocol for CalDAV subscriptions that works on Apple and Linux

Unfortunately the user story is bad on Android, where Google hates any standards.

Edit: You can test out a recent project I did with webcal at https://captnemo.in/blr-habba/

Slightly related: If you're self-hosting your email, you can use autoconfiguration[1] and autodiscover[2] to help your users set up Thunderbird and Outlook correctly.

[1] https://wiki.mozilla.org/Thunderbird:Autoconfiguration

[2] https://support.microsoft.com/en-us/topic/outlook-2016-imple...

Once you’ve installed a mobileconfig profile on iOS, is there a way to view its source?

iTunes unencrypted backups have the files, so you need to take a full backup and extract the relevant files: https://apple.stackexchange.com/questions/292869/how-can-i-e...