From the sounds of things it’s not actually the Android 15 update that’s causing the problems; it’s actually setting up the Private Space feature. At least three users have come across the same issue and posted about their experiences on Reddit. Two of those users had a Pixel 6, while a third had a Pixel 6 Pro.

Multiple users on Reddit have reported their Pixel 6 phones suddenly dying after installing the Android 15 update. In at least two cases, the issue occurred after enabling the new "Private Space" feature,

Mobile phones are like little treasure chests of sensitive personal data, so Android has introduced new anti-theft measures to its Android 15 for Pixel update.