One of the few remaining ubiquitous open formats of the web. Unfortunately Apple still refuses to recognise *.ics files unless they're included as a Mail attachment.
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Not true. I just tried it on an iPhone 14 with iOS 18.4, and it works fine.
You tap on the ics icon, and it opens up the details. Tap "Done" and the event lands in the calendar you specified.
Contrast this with opening the same .ics file from an email attachment or viewing it online, which presents an "Add All" button.
Not that I ever get a .ics file from anywhere except my emails, but it would be nice to be able to actually use them if I have them.