The iMessage feature is a parental control feature where kids over 13 receiving on-device classified sensitive images have to click through to unblur them, and kids under 13 will do the same and also have their parents receive a notification that such an action was taken. The parent in any case does not receive a copy of the image. The EFF described it as such:

“Whatever Apple Calls It, It’s No Longer Secure Messaging”

and

“Apple is planning to build a backdoor into…its messaging system.”

The Center for Democracy and Technology who wrote this letter they have co-signed said:

“Plan to replace its industry-standard encrypted messaging system creates new risks in US and globally”

I, respectfully, don’t see much evidence that these are consensus views. Furthermore, I don’t see how you can characterize this feature as a back door without believing safe browsing checks on links received in iMessage is an encryption back door.