Is it safe to create a topic for every user?

Question

Notifications

There was a need to send push notifications to each individual user, but the device_token is not stored in the database of my application. It was decided to create a topic for each user, for example: "topics/{user-id}" to identify a specific user and send a notification to such a topic. Actually the question is how NOT secure it is and what does an outside user need to intercept these notifications?