In German class in 1999, we were also taught "tschüß" was correct. But this was immediately before the Schreibtrechtreform and so I also write "ißt" instead of "isst". So there's a bit of variation because it's an informal word and therefore a bit variant in certain dialects.
I know that after the Rechtscheibreform (reform of the entire German language in the late 90's), the esstset (that's the letter that kind of looks like a capital B but actually represents a double S) is only used following long vowels. Prior to that it was also used at the end of sentences or if it was only followed by on letter.
While that may be true, I think that would cause mass confusion in a language program. In Quebec (Montreal), some French and English speakers use "Ciao" to say goodbye, but I would not expect it to pop up anywhere but in the Italian section of Duolingo. Just my opinion.