Hack and Slash, Megadungeon play, with few or no NPCs. Let us set the DM issue aside, as I did in the post where Oofta declared this style of play "Not DnD" (because yes, I did set it aside. I had already shown this style could work without a DM, and was told this style was not DnD and that was what I was defending in that post)
What rules are we changing?
Still rolling the same dice.
Still making characters using race, Class, Background.
Health, Class abilities and AC are unchanged.
Skill Check rules are unchanged.
Monster statblocks from the Monster Manual, as they generally are
Still using the DMG to adjudicate stone, wood, ect.
Still using all the spells in the PHB, though some are obviously less useful without NPCs.
Sure, there are plenty of rules we aren't using.... but no rules were changed. So if we are playing DnD, using DnD rules, why isn't it DnD?