Well, first of all, back then, I figured that all of Jim Henson and Richard Hunts characters had been given new performers, even though I didn't see all of their characters in new productions. I also figured that Richard Hunt had died, but didn't see anything that specifically said he was dead (besides the opening dedication in The Muppet Christmas Carol, and even then I thought I was just guessing that "In Loving Memory of..." credits meant that the people were dead). Second of all, back then I didn't know what Sesame Street characters Richard Hunt performed. I knew of some regular characters who Jim Henson, Frank Oz, Jerry Nelson, Fran Brill, and Caroll Spinney had performed on Sesame Street, but not Richard (I had looked at the album credits on the tenth anniversary album, which listed Richard Hunt along with Christopher Cerf and Jeff Moss as the Monotones, but I didn't know who they were at the time, and I also looked at the album credits for the Jim Henson: A Sesame Street Celebration album, which said that he performed Eddie, a one-shot character from the Captain Vegetable song). I didn't know that Richard Hunt performed Gladys, Sully, half of the Two-Headed Monster, Placedo Flamingo, Forgetful Jones, Sonny Friendly, or Don Music until later.
Having said that, I didn't notice that those characters were no longer being used on the show in new material. Sully did appear in some new material, but he didn't talk, and both Forgetful Jones and Gladys made occassional background apearances in the years after Richard Hunt's death. At the time I couldn't really tell when a segment looked like it was made in, and didn't have any idea if a skit was new or not (I did notice a difference in film quality and different looks in characters over the years, but didn't know that the film quality and differnece in character appearances often had to do with when they were produced). At the time, the only character who I had noticed wasn't being used anymore was Bruno.