I believe products already do this. A good few years ago I was asked by a guy why his plugged-in HP laptop was telling him it “wasn’t charging”. I looked up the docs and discovered that between 95-100% it would turn on/off the charging as required.

I’ve heard separately (no source, sorry [edit: see below]) that Apple products also do this, they just don’t confuse the user by telling them that it’s happening. I wouldn’t be surprised if HP et. al. have since changed their user notification behaviour, too.

Source: forum posts going back almost a decade. https://www.google.com.au/search?q=apple+laptop+stops+chargi...

Wow, that's a pretty neat feature. I think there are many people who leave their phone to charge during the night, only for it to 'fast charge' up in 1 or 2 hours and then sit at 100% for the rest of the night.

Here[1] is the screen.

Here, the time isn't based on an alarm (none is set), but on when the charger was disconnected the previous morning.

[1] https://i.imgur.com/1EywDA0.png