Every user has an unique ID.
Every message sent on Telegram could be related to an user account...even if it has been deleted (the user ID remains the same even after the account is deleted)!
In addition user IDs could be used (and ARE USED) to track groups a particular user has joined (that's even easier with public groups), name & nickname user changes etc.
The data you can gain throughout user IDs is astonishing and it could be sufficient to profile a person.
(these two solutions aren't mutually exclusive and could be implemented together)