The data is said to have been used to attempt to tie anonymous users of messaging apps to specific Apple or Google accounts.
So it's not about the notifications or even necessarily the data the app handles; just that there's an apple ID or google ID they're pinging to see who it is.
Today's lesson is: Never use your apple ID or (ugh) google ID for anything important. If you can not use either for anything, great, but we all know we're not international super spies and sometimes you just want to play a card game or something. Still. If someone's unaware that smartphones are tracking devices they should probably know that now.
I'm amazed that Apple was prohibited from saying anything until now.