I'm gonna have to ask - why was F_SKY1 removed altogether? That is to say, it behaved exactly the same in the original DOOM games, but THEY had a flat with the same name so you could assign the sky texture to a floor or ceiling in various editors with ease. SRB2 lacks that, and I think that's pretty lame.
For now, I made one for myself out of GFZSKY, and it behaves as it ever did - although now I can actually select it from the texture browser in DoomBuilder, instead of having to continuously type it in manually. In other words, I'm covered, but it still just boggles me why it's been missing for so long.