It's all speculation, but they _seem_ to track a large amount of data to try and check for "new profiles" that are old users starting a new account, supposedly for 3 months after you delete your account. These are the ones I've seen commonly talked about.
- Phone Number
- IP
- Device IDs
- Email
- Photo hashes
- Photo exif
- Facial recognition
- Linked Facebook, Instagram, Spotify accounts