I'm trying to figure out if Voodoo Demons spawn after defeating The Wall of Flesh? If Voodoo Demons do spawn, will their spawn rate be the same?
Yes, they still spawn after defeating the wall of flesh.
The spawn rate is slightly lower after you enter hard mode and then slightly lower again after you start to beat hardmode bosses. This is because of the new types of enemies that can spawn in hardmode, lowering the percentage voodoo demons spawn.