They would have used the "ghost user" strategy.

> "Perrin now offers a different framing. “Article 8 ter, which I had adopted, was not at all aimed at obtaining encryption keys but at introducing a ghost participant into a conversation before encryption,” he says. The “ghost participant” approach, sometimes called a ghost user proposal, was floated by GCHQ in 2018 and rejected by every major privacy organization, civil liberties group, and security researcher who looked at it. The idea is that the platform silently adds a third recipient, an invisible intelligence agent, to a supposedly two-person conversation. Users never see them. The encryption technically still works, except that one of the parties is the state."