ZDoom

[quote="Gez"]Nightmare mode never disabled iddt nor idclev. Those are the two cheats you can use in vanilla Nightmare skill. This has been true in all official versions of Doom ever since the Nightmare skill was added in the first place.
http://doomwiki.org/wiki/Skill_level[/quote]
Maybe you're right, but I don't understand why this cheats aren't disabled in Nightmare skill

Confirmed (not that it really needed confirmation) that idclev## and iddt both work in doom2.exe in Nightmare difficulty.

[quote="Caleb13"]Just out of curiosity - how this behaves in original Heretic and Hexen DOS executables?[/quote]
Interesting.
Heretic:
I was able to get the map cheat (ravmap) to work at the toughest difficulty level (Black Plague) but not the level warp (engage##).

Hexen:
When starting a game with any class, I could not get either the map cheat (mapsco) or the level warp (visit##) to work on the toughest setting for the class. However, if I started a game on a lower skill setting and used the map cheat, then started a new game on the hardest setting, the map cheat would still be active. I suspect that this inconsistent behaviour is a bug/oversight.

But in Zdoom, IDDT is disabled - any attempt to use it in Nightmare results in "SV_CHEATS have to be enabled" line in console.

Just out of curiosity - how this behaves in original Heretic and Hexen DOS executables? I became so reliant on Zdoom that I don't even have them on my drive anymore...

Nightmare mode never disabled iddt nor idclev. Those are the two cheats you can use in vanilla Nightmare skill. This has been true in all official versions of Doom ever since the Nightmare skill was added in the first place.
http://doomwiki.org/wiki/Skill_level

Yup.

Doom2.exe:
Start game on default difficulty.
Go to automap
type IDDT: full map appears (or IDDT IDDT to see things too)
bring up menu
start new game on "Nightmare"
check automap: full map still visible.

Indeed; Restarting, ending or loading a game in Doom 1.9 does not reset the automap.

Needs to be checked with Doom.exe, to see if it works there, too.

Not sure if a bug or a feature, but nobody mentioned it yet, so... it is possible to have full map cheats enabled on the highest difficulty. You have to start a new game on some lower difficulty and enable the map cheat. If you then start a new game on the highest difficulty, the map cheat remains enabled and you can see full map. It also works if you load some hardest savegame. I first noticed it in Heretic, but it is the same in both Dooms, too.