Is asking for the syllabus as a student before class starts frowned upon? Is asking for the syllabus before class frowned upon?
I usually ask because I'm trying to figure out what we're going to do and whether I have interest in the subject.
It should be perfectly acceptable (nearly) everywhere. You may not get it, for a variety of reasons, but it is fine to ask. Note that some places have it online, so you might look there also.
Even an older syllabus might serve your purposes.