Zandronum

I'm trying to make a ball that would be batted around a room, horizontally towards a goal. When it hits the goal, it needs to disappear.

I've set the floor texture at the goal to the sky and that didn't kill it... and I also tried stretching the sky downward and that didn't work either.

How would I make the projectile disappear when it hits the sky? Or disappear when it crosses a linedef?

Try +SKYEXPLODE

Forgive my ignorance as I'm not much of a mapper, but isn't there a 'projectile crosses' or similar activation type for linedefs? You could use that with Thing_Remove or an ACS call with whatever you need.

As for the sky texture, I think your best bet is calling a script with an 'Actor hits floor' thing (assuming it works for projectiles - I've never tried it) and checking an actor property that can safely identify your projectile, such as seesound or deathsound, then removing it.