Maps that deliberately use weird way sky works in Doom.

[ViolentBeetle]

Once upon a time I made MAP31 for Modest Mapping II project. It's a maze that has lowered sky ceiling as walls, causing things and geometry to disappear behind literally nothing.

However, I never seen any other map that does something like this intentionally (There's maps that don't utilize sky hack well and do it by accident, including IIRC Citadel from Doom 2).

Here's an example from aforementioned map, but there's other concepts that wouldn't be possible if the game was actually 3D.

[Shepardus]

The beginning of 50 Monsters MAP31 has crushers that appear as sky, but they don't render correctly in hardware renderers such as PrBoom+/dsda-doom's (where they're completely invisible) and GZDoom's (where the sky walls appear but the ceiling appears where it shouldn't).

[head_cannon]

MAP08 of Three's A Crowd uses skywalls quite extensively, putting you in a maze that's effectively invisible to you. Since the skybox is drawn for both the flats and the walls, it's like you're just walking in the sky and have to use the scattered sprite decorations as landmarks to navigate by (which of course disappear as soon as you go around a corner, making for a very disorienting environment). And then it forces you to try to evade an Arch-Vile under those circumstances! It's an aggravating gimmick, but a memorable one.

Edited January 2, 2023 by head_cannon

[Shepardus]

There's also the wall behind the chaingunners in Plutonia MAP15.

[Book Lord]

I'm not sure what kind of effect you're exactly looking for, if it's just non-orthodox use of sky texture hack or a more specific effect. 

 

The Casali brothers used the sky texture to hide and then reveal portions of their maps, as seen in TNT:Evilution MAP32: Caribbean and the aforementioned Plutonia MAP15. A similar trick was used by Denis Moeller in Memento Mori MAP13: The Inmost Dens 2.

 

For weirder use of sky, I recall the invisible platforms of MAP12: Strange Universe of Number One Kill The Next Generation.

Edited January 2, 2023 by Book Lord

[SCF]

It's also used in Map28 of 30 Monster Challenge by @LPad

 

[Fairen]

(BEHOLD) THE FIRMAMENT in The Sky May Be, which used skywall for a mind-screw-y maze room.

[Roofi]

I used the yellow sky texture of the first episode of 180 minutes pour vivre in order to imitate golden ore (left part of the screen)

 

180mpv map 08

 

[Gez]

dead.wire does pretty interesting tricks with its static noise sky. Entire areas of the map are "reclaimed" from the static void as you repair the base and it creates a very eerie effect.

[Gez]

Oh, and this weird experimental thingie (only works in ZDoom I think; effect is busted in GZDoom).