Hyphens, En Dashes, Em Dashes
Q. It’s 2020. Can we please stop using a hyphen in “dropdown”?
A. According to the Apple Style Guide (dated December 2019), the term shouldn’t be used at all:
drop-down menu. Don’t use; use menu.
The Microsoft Writing Style Guide allows it, but only for an audience that includes developers:
It’s OK to use drop-down as an adjective in content for developers if you need to describe the type of UI item or how it works.
Apparently the user interface works by a sort of magic whose secrets are revealed only to magicians. Part of that magic may have something to do with the hyphen in “drop-down,” so it’s probably best not to meddle. On the other hand, Merriam-Webster lists “drop-down” and “dropdown” as equal variants for the noun form (the adjective form is always hyphenated), so maybe there’s hope for you. Just don’t tell Microsoft: according to the Microsoft guide, the noun form is verboten.