Downgrading from iOS 27 is no longer possible, as Apple will no longer be signing the previous version, iOS 26.2.2, after a week of iOS 27. Softwares that are signed go through verification on the server side when a new version of iOS is installed. You will not be able to install the update if the version does not pass the verification check.
Apple does this to protect its customers by preventing them from downloading less secure and outdated versions of iOS because new versions deal with previous vulnerabilities in security.
You can remain on iOS 26 if you have not updated to iOS 27 but when you update you cannot go back as iOS 26.7 is an update that is over the air with restore files unavailable.
iOS 27 brings the new revamped Siri and patches in security to protect its customers.
