Personally, I’m skeptical, especially of the following:
Who does this apply to?
The new measures are only available to children who are members of a shared iCloud family account. The new system does not work for anyone over the age of 18, so can't prevent or detect unsolicited images sent between two co-workers, for instance, as the recipient must be a child.
How can I opt out of this?
You don't have to, parents of children to whom this might apply must opt-in to use the feature, which won't automatically be turned on when you update to iOS 15. If you do not want your children to have access to this feature, you don't need to do anything, it's opt-in, not opt-out.
Doesn't this compromise iMessage's end-to-end encryption?
Apple's new feature applies to the Messages app, not just iMessage, so it can also detect messages sent via SMS, says John Gruber. Secondly, it should be noted that this detection takes place before/after either "end" of E2E. Detection is done at both ends before a message is sent and after it is received, maintaining iMessage's E2E. It is also done by machine learning, so Apple can't see the contents of the message. Most people would argue that E2E encryption means only the sender and the recipient of a message can view its contents, and that isn't changing.