Apple has stopped signing the iOS 17.5, meaning iPhone users can no longer downgrade to the 17.5 version of the iOS. After May 20 once iOS 17.5.1 is released, users will be unable to downgrade to the previous version.
This is not a new thing for Apple to do, since it usually happens when a week or two has passed after a new update of the iOS rolls out. When an update is not signed anymore the user is no longer able to install that version of the iOS on the iPhone via verification check on the server side.
Apple does this to persuade the user to have their operating systems updated with the most recent security improvements. The iOS 17.5.1 addressed a bug that causes deleted photos to pop back up on an iPhone. The latest update is available to the public and the only released version that is accessible.
