Not available for me either but is when I create a NEW event.
I’m also linked to Microsoft Live as my email provider ( that’s also how I sync across devices) and strange, if I go into the “calendar” tab of the OUTLOOK App on my iPad the “All Day” button IS there if I try to edit an existing entery.
edited to add ….
All of the above was checked on my Ipad (iPadOS 15.2), subsequent testing on my iPhone (iOS 15.2) and the button IS there in Edit ??
Looks like a bug in iPadOS 15 ?