Someone left a link in a comment to a site that explores the complicated issue of the origin of the word syllabus and the related issue of what its plural form should be.
Iโll leave that subject to be debated by the English etymologists and Latinists among you. I shouldnโt have to worry in the mean time: since one course only ever has one syllabus at a time, I shouldnโt ever need to use the plural.