I do have one question: Didn't Herbert's hair and mustache get recycled on Mr. Essex later in the 1980s? I know Herbert's eyeglasses did get used on many other characters, including Mr. Essex, Don Music and the Lavender Muppet guy who was tending to his garden when the flowers started to talk whenever the guy was away, to name two characters. Didn't Herbert's glasses also get worn by Dr. Nobel Price?
I find that Mr. Essex was basically a clone of Herbert Birdsfoot, except for the nose and wardrobe.
Also, the name Herbert (or a variation) did get recycled later on with Herbert Hare in the letter H Alphabet Chat segment, and Herb the Dinosaur later in the 2000s. Herbert Birdsfoot was rarely seen in the 1990s, it seems - the final showing of one of his segments where he posed as a teacher was in Season 28, if I recall. But his official final appearance on an actual modern broadcast of "Sesame Street" was a cameo appearance in the Ernie & Bert sketch about a surprise party, which was aired in Season 32, in 2001.