Stealth Wads?

Are there any wads that require sneaking past monster, hiding from monsters, quick-and-silent kills, or other stealth tactics?

Haven't heard of those, although I've seen some attemtps.
It's really hard to do something like that in Doom engine.

Taking into account that monsters don't really give a flying you-know-what whether you're sunbathing on a desert in high noon, or standing in a pitch black corner; the only sound they recognize is the one you make while firing (or swinging your arms around <FFS>), and they don't alert each other...

...it's still possible. Very hard, but possible. It would require great scripting skills, and a lot of attention while planning the map/monster layout. I'd love to see something like that myself.

I'm not sure there are any wads that are specifically designed that way, but there certainly are maps where it is a good strategy to stay quiet (i.e. not firing) for a while or in particular areas, and to take care that certain monsters don't see you, etc.

Strife (Which is not a wad but a game based on the Doom source code) feels like your playing in stealth a lot. Than you set off the obligatory alarm and everything changes.