It is sign language (or speech).
Yes. Sign Language is speech.
Rhetorical Critics will tell you that anything is “speech” if the purpose is to convey a message. That includes things like architecture, art, clothing, or no clothing. Certainly women who go topless are attempting to convey a message. Trying to figure out what that message is can be problematic. The next question is “is this protected speech”. My take is that the Founders were protecting political speech. So unless these women are running for office on a platform of [insert joke here] it’s not constitutionally protected speech. IMHO