>>96482772
>That's already addressed by the higher XP requirements to level up to the 9th
Wrong.

Both in OD&D and AD&D, when a character or party faces challenges that are too weak, Gary (play be upon him) instructs you to reduce the XP awarded proportionally to the "power ratio". At least in the DMG, that is required reading, this applies to XP from defeating monsters, overcoming tricks and traps, and XP from treasure as well.

This is NOT addressed by the higher XP requirements to level up because it's a rule designed to punish what today we would call farming.

Picrel is from DMG page 84.