“According to the report, Wyden's letter said a "tip" was the source of the information about the surveillance. A source familiar with the matter confirmed that both foreign and U.S. government agencies have been asking Apple and Google for metadata related to push notifications. The data is said to have been used to attempt to tie anonymous users of messaging apps to specific Apple or Google accounts.”

https://www.macrumors.com/2023/12/06/apple-governments-surveil-push-notifications/