As far as I know, and I could be wrong, his last appearance was in the lorretta switt episode of The Muppet Show, from 1980. I have read that the Thog puppet/ suit wore out and deteriorated so badly that it was thrown away. I'm not sure when he wore out, but I have a feeling that it might have been in the early 1980s, since he wasn't in The Muppets: A Celebration of Thirty Years, which was made in 1985. Since he was a full body character, probably really expensive to build, and such a rarely-used character, it's not likely that he will be rebuilt anytime soon, but as Ken Lilly from palisades often said about the many questions asked about the Muppets action figure line, "anything is possible".
I hope that Disney does eventually see a need to rebuild him dispite his obscurity. Maybe if Disney decides to make an animated (or computer animated) Muppet production they can include him (it should be easier to animate characters than to build them).