New Apple security feature reboots iPhones after 3 days, researchers confirm

This "novel" feature is already supported by GrapheneOS and set to trigger after 18 hours by default, with the option for the user to adjust it to their preference. There is no good reason to force the choice of 72 hours on everybody. That's a user-hostile design decision.

This is an essential feature for my personal GrapheneOS phone. I only tend to use it once or twice a day most days, which means it is usually freshly rebooted every time I go to use it.

I remember reading somewhere that many new exploits in the mobile space only exist in memory and are thwarted by a simple reboot, including the infamous Pegasus spyware.

Graphene might be great, but the Google specific hardware that is the only thing it will run on might not be as trustworthy.

Graphene is not limited to Google hardware, it's just that Google hardware meets their hardware requirements... (https://grapheneos.org/faq#future-devices).

AFAIK (from observing GOS comm channels) verified boot (alternative OSes but even the mechanism itself since some OEMs customize quite a bit), hardware rate limiting and timely security patches (which include modem firmware, preloaders - i.e. hardware) are the main reasons other devices are not supported.