The usage of "ß" (also known as “ess zett” or “sharp s”) changed following a major German orthography reform in 1996. Prior to the reform, the ß was used following a long vowel or two consecutive vowels (also known as diphthong) as well as at the end of a syllable or before consonants. The conjugation of müssen was, for instance:
Correspondingly, the infamous German subordinate clause conjunction dass used to be written as "daß" as well.
After the reform and a bit of political back and forth, the first rule still applies, while the second was dropped. So nowadays the ß tells you that the vowel before is long, whereas "ss" is used when it is short
e.g. Straße (long vowel)
The ß is also used after two consecutive vowels.
So contrary to popular belief, the the ß hasn‘t disappeared from the language, its use has just been changed.
On our German Language Blog "Auf Deutsch, bitte!", you also find posts on very German things to do, long German nouns, why the English word fun is used in a different way in German, why Am Morgen and morgens are not the same, we explain the category of weak nouns in German, the difference between kennen und wissen, and we provide reading comprehension exercises at A2 German, B1 German, B2 German We also warn you about the top 5 mistakes in German and tell you how to avoid them, and we have articles that are of interest to language learners more generally, such as a review of online dictionaries Linguee, dict.cc, dict.leo, and Collins, and a comparison between Duolingo, Memrise, Babbel, and Busuu.