I'm probably the last person to notice this, but Garmin actually added the option for disabling the lap key at some point. If anyone at Garmin is reading, thanks for listening to us! :) This is something that I legit thought would never happen.
When you disable the lap key (per-activity setting), Back takes you back to the watchface. It's perfect for a race or a long run when you probably don't want manual laps.
Previously, it was really baffling to see that Fenix 5 had this option and not the 935. I did notice that disabling the lap key also removes the Lap option in the pause menu, which is probably not what some people wanted, but it doesn't bother me. Anyone who still needs the occasional manual lap can set up a two-button hotkey which should be impossible to trigger by accident.