Technology

Apps are easier, and they can design the app so they they query a list of all your other apps. They'll know if you used any encrypted messaging apps, so later they know to search your chat logs and you cannot have plausible deniability, since they'll know which exact apps you used (unless you use a second phone for your encrypted messaging apps).

If they make it mandatory, using a flip phone is gonna be breaking the law.

Edit:

Article says:

Using a mobile application that all foreigners will have to install on their smartphones, the Russian state will receive the following information:

• Residence location
• Fingerprint
• Face photograph
• Real-time geo-location monitoring

I think they are trying to verify that you actually have your phone with you, not just handing it to someone else and then sneak into a protest to create an alibi.

I speculate they will eventually randonly ping your phone and you're expected to tap the notification to verify you are indeed the person holding the phone via facial recognition. Like a "check-in" with a probation officer type of thing.