New iOS 27 software update issu

While you can't downgrade from iOS 27, I can tell you that iOS 27 did not remove the ability to set up a calendar event and choose recurring options, just like you could in just about every previous calendar on iPhone. I just set up an event as a recurring event in the calendar on my iPhone to make sure it still works, and indeed it does.

Perhaps Force Restart your iPhone and try again --> Force restart iPhone - Apple Support

You also might want to read this --> Create and edit events in Calendar on iPhone - Apple Support

Under event, select the date box.

You get a selection called repeat.

it defaults to never, select it to set when it repeats.

How can I undo any iOS upgrade?

This is not possible. Apple has never supported downgrading.

I'm absolutely, 100%, positively sure. I just started a new Calendar event for a person I don't know, but here is the screen shot:

I'm glad you now know how to find it and you're very welcome.

Welcome again!