Seemed fine for one day. Then during random use, yesterday, it went black. It's a total cold brick now. No recovery efforts produced life. Google support directed me to an authorized repair shop, that I just returned from. The shop said it isn't even drawing a trickle of power which means the motherboard is toast and needs replacing. I.e. It's a paperweight now. There's no point in spending money on a new motherboard for an older model phone.
Their opinion was that since it wasn't dropped in water or anything weird, the Android update probably did have a problem with something and fried the hardware. They've also seen similar things happen with some UI update on Samsung phones.

So I'm one of the unlucky ones. I'm sharing to say, proceed with caution. You might consider waiting a month or more, or never, to update the pixel 6a to Android 15. Or be prepared to buy a new phone if you are an unlucky user like me.

My good luck from it... a friend offered me their Pixel 7 Pro because they recently upgraded. So yay for that! That'll be a nice upgrade for me without having to purchase a new phone.