It's fairly easy to "enable" a "dumb mode" on a smart phone. iOS supports "guided access", where you can basically turn off everything, behind a passcode. There

It's fairly easy to "enable" a "dumb mode" on a smart phone. iOS supports "guided access", where you can basically turn off everything, behind a passcode. There's also screen time, which lets you limit/disable anything, also behind a passcode. But, this requires that someone else manages your phone.