They haven't been seen in new Muppet productions since "The Muppet Show" ended.
Actually, Kermit (Jim-era) gave us the answer to that one:
In the Linda Lavin(?) TMS episode, where the gang stages a "This Is Your Life" for Kermit's birthday, they bring back W&W after two years, who proceed to launch a major guilt trip on their host ("We've been keeping busy, ever since you
fired us...")
Kermit lets them do one more number out of pity, they burst into "Sweet Mystery of Life", and...
Kermit: "...NOW I remember why I fired you!--Out, out, get out!"
