After painstakingly removing the rear panel from the iPhone 8 intact, we decide to go hard on the heat, just in case that's the secret sauce.
Spoiler alert: it isn't. It looks like Apple's adhesive of choice is pretty impervious to heat. If anything, it softens the metal frame a bit, but it doesn't seem to do much to the glue.
This time, we totally broke the panel on purpose, to try out a more realistic situation of removing a broken panel. The verdict: don't drop this phone. Removing a broken panel is much more difficult than getting it out in one piece.