Cider9986 answered for Android, so I'll throw out a suggestion for iPhone.
Assistive Access on iPhone might be an option for people looking for something drastic. Turning it on is simple, but it's pretty brutal and a bit crude in some ways even compared to a feature phone. Your mileage will vary! It's something I often suggest, and never quite recommend.
You pick the apps you want access to, and the permissions each should have, set a password, and then when you turn Assistive Access on, the phone reboots into a very limited mode. You can have every app you want, but when I've played with it, I've still found it felt too limited for daily use. Maybe I wouldn't find that if I was at the point of buying a feature phone. I can't remember what frustrated me, except that I remember being pleasantly surprised by how much worked, and frustrated by some basic things.
As an example, I was impressed that I could turn on and off a VPN through an app, even though I couldn't see the status of it outside the app. On the other hand, the location permissions felt buggy, and the locations permission changes in Assisted Access mode seemed to mess with the settings in the normal mode too.
Live in user profile, keep owner profile with appstores. Push apps that are distractions free into user profile.
Use ADB to remove the built in browser because you can't just delete it or not install it because it's a system app. On GOS it's the only system app that is distracting, but I can imagine other phones might have others. Same principle, just remove it with ADB from the user profile.
Never install an app store in the user profile.
Owner profile password mitigation. You have a few options. Make it way too long to easily type and memorize it, write it down on paper and put it away in basement/attic/friends house, give it to a friend, give part of it to a friend(so they can't unlock the owner profile, only you can, but only if you ask them so huge friction).
Personally, I just have a super long passphrase memorized and that's enough too make the friction large enough. And it's really peaceful on the user profile.
Result. Without the owner password, I am in the user profile and I can't browse the web(HN) or install a distracting app like TikTok or install a new browser. If I want to update an app or manage the device or when the device restarts
Back when I was on iOS I used Apple Configurator which is Apple's MDM solution. You need a Mac it borrow one.
You remove Safari and disable installing apps. This is the guide I followed. Pretty sure your have to factory reset your phone first.
So, to install new apps you have to connect the iPhone to the Mac and optionally add a password.
MDM is supported by Apple, uninstalling the browser is not recommended by GOS developers, but I haven't had any issues. Soon, GOS will support MDM, so hopefully that will be an even better solution.
Cider9986 answered for Android, so I'll throw out a suggestion for iPhone.
Assistive Access on iPhone might be an option for people looking for something drastic. Turning it on is simple, but it's pretty brutal and a bit crude in some ways even compared to a feature phone. Your mileage will vary! It's something I often suggest, and never quite recommend.
https://support.apple.com/en-sg/guide/assistive-access-iphon...
You pick the apps you want access to, and the permissions each should have, set a password, and then when you turn Assistive Access on, the phone reboots into a very limited mode. You can have every app you want, but when I've played with it, I've still found it felt too limited for daily use. Maybe I wouldn't find that if I was at the point of buying a feature phone. I can't remember what frustrated me, except that I remember being pleasantly surprised by how much worked, and frustrated by some basic things.
As an example, I was impressed that I could turn on and off a VPN through an app, even though I couldn't see the status of it outside the app. On the other hand, the location permissions felt buggy, and the locations permission changes in Assisted Access mode seemed to mess with the settings in the normal mode too.
I didn't use an article, I just followed the principles and had an LLM do the android debug bridge commands.
Here is an article I found later which did the same thing as me.
(https://jordanherzstein.neocities.org/posts/adb_vanadium/)
For Android basically:
Live in user profile, keep owner profile with appstores. Push apps that are distractions free into user profile.
Use ADB to remove the built in browser because you can't just delete it or not install it because it's a system app. On GOS it's the only system app that is distracting, but I can imagine other phones might have others. Same principle, just remove it with ADB from the user profile.
Never install an app store in the user profile.
Owner profile password mitigation. You have a few options. Make it way too long to easily type and memorize it, write it down on paper and put it away in basement/attic/friends house, give it to a friend, give part of it to a friend(so they can't unlock the owner profile, only you can, but only if you ask them so huge friction).
Personally, I just have a super long passphrase memorized and that's enough too make the friction large enough. And it's really peaceful on the user profile.
Result. Without the owner password, I am in the user profile and I can't browse the web(HN) or install a distracting app like TikTok or install a new browser. If I want to update an app or manage the device or when the device restarts
Back when I was on iOS I used Apple Configurator which is Apple's MDM solution. You need a Mac it borrow one.
You remove Safari and disable installing apps. This is the guide I followed. Pretty sure your have to factory reset your phone first.
https://redd.it/1731ozp
So, to install new apps you have to connect the iPhone to the Mac and optionally add a password.
MDM is supported by Apple, uninstalling the browser is not recommended by GOS developers, but I haven't had any issues. Soon, GOS will support MDM, so hopefully that will be an even better solution.