Slymenstra Hymen's disappearance from the GWAR stage shows and band storyline was never explained. Then again, the Danielle Stamp (who plays her) makes occasional appearance in character at shows, also without explanation. Same goes for Sleazy P. Martini, their manager, who is similarly gone from the regular show and storyline, but will occasionally don the pompadour for a show or two.
Techno Destructo at least gets a little story of his drop in/drop out nature; Flattus officially "went back to his home world" when guitarist Corey Smoot who played him at the time died (instead of the new band member playing the character as they had in the past).
Barely a year after adding the character Vulvatron, in the wake of Dave Brockie's death, the singer who played her (Kim Dylla) was fired, apparently for drinking too much, and the character unceremoniously disappeared.