clearstream
(He, Him)
All or almost all D&D mechanics can easily be made diegetic: I simply pretend my character can know about them. Rather than saying as player that my character says "I'm going to climb this wall" or just sets about climbing, I say as player that my character says "I will roll a d20 modified by my Strength ability and Athletics proficiency against the DC to climb this wall" or just rolls d20, adds the appropriate modifiers and compares that with the DC.But yes! This does in fact mean that essentially every mechanic used in D&D isn't diegetic and almost certainly can't be made so. That shouldn't really be a surprise. We are still primarily using slight variations of mechanics invented around 50 years ago primarily to let some wargamers get up to some silly shenanigans with swords and sorcery. They weren't meant to be diegetic at all; they were simply meant to be adequate to get useful information across.
My character could have lengthy in-world conversations about the mechanics. The d20 at the table could be taken as a prop associated with a diegetic d20 that my character carries in their pocket, ready to roll.