Another underserved group of users is people with ADHD. I think a "minimize distractions" mode at the OS level would be very welcome by a lot of people.
The OS does have focus modes as of a year or two ago. You can set it up to only allow the apps and notifications you want.
Plus you can have multiple to suit the context you’re in
As @dagmx said above, we've already had a great bunch of features added in the last 18 months, time and geo sensitive focus modes that can lock a device to only certain apps and/or certain features, notification center batches notifications up instead of them being drip fed