Maybe the concern is that Firebase still learns that user A and user B receive notifications back and forth at around the same time, interspersed with periods o

Maybe the concern is that Firebase still learns that user A and user B receive notifications back and forth at around the same time, interspersed with periods of no notifications for both If A and B talk regularly, there might not be many other C who by coincidence exchange notifications at the same time So you might be vulnerable to a sort of traffic analysis