PastaGorgonzola

It's rather vague to me too, the most helpful summary I found was this one:

In general, the condition applies when:

• The processing isn’t required by law, but there’s a clear benefit to it;
• There is little risk of the processing infringing on data subjects’ privacy; and
• The data subject should reasonably expect their data to be used in that way.

So "we don't have to do this, and most likely it won't be privacy sensitive, and you probably already know we want to do this, but you can still opt out"

Source: https://www.itgovernance.eu/blog/en/the-gdpr-legitimate-interest-what-is-it-and-when-does-it-apply

I'm not convinced by this argument: at the back of the phone is a built-in LED (used as the flash). Which could be used for notifications too.
Phones with OLED screens could use part of the screen as a notification as well. Both of these can be accomplished in software. Currently you have to notice that something happens as it happens, otherwise you need to at leas activate the screen. The notification LED was useful in that you could glance at your phone and see if you missed something.