It's been used by state actors for years. May as well publish the API for it. These aren't rare and special flowers that are each artisanally discovered after searching for a million hours. And it is nicely packaged in an app that is used for something simple like text messages.
"An Israeli spyware company has reportedly cut access to its clients in Italy following allegations that its product was used to target critics of the Italian government.
"The move comes after WhatsApp alleged last week that spyware made by Paragon Solutions was used to target 90 WhatsApp users in two dozen countries, including journalists and civil society members.
"Italy's government confirmed in a statement on Wednesday that seven mobile phone users in the country had been targeted by spyware on WhatsApp, calling the incident "particularly serious"."
Why are you quoting an article about WhatsApp after making a claim about iMessage and what Apple builds in?
(Not that the article has any evidence supporting the same claim about WhatsApp, either. In fact, the article describes WhatsApp/Meta claiming to have "disrupted" the spying and reporting it to the Italian government.)
The feature is not named "back door" or "master key". It's a feature of iMessage with various names such as "zero click".
Also note they have created features before that provides law enforcement access to data at different stages of a pipeline.
https://en.wikipedia.org/wiki/IMessage#Security_and_privacy