Apple today stopped signing iOS 26.4, so iPhone users who have updated to iOS 26.4.1 are no longer able to downgrade to the earlier version of iOS. iOS 26.4.1 came out a week ago.
When software is "signed," it means it can pass the server-side verification check that Apple does when a user downloads a new version of iOS on an iPhone. An update that's not signed can't be installed because it won't pass the verification check.
Apple does not show users earlier versions of iOS after an upgrade has been released, but when software is still signed, it is possible to downgrade with the macOS Finder app on a Mac or the Apple Devices app on a Windows PC.
Unsigning software prevents Apple customers from installing outdated, less secure versions of iOS, and Apple typically stops signing an update a week or so after new software comes out.
iOS 26.4.1 fixed iCloud syncing issues and Stolen Device Protection on enterprise devices. While iOS 26.4.1 is the current publicly available version of iOS, Apple is also beta testing an iOS 26.5 update.