The Nevi'im sometimes got prophecies and wrote them down in Nevi'im or Kesuvim, and from those prophecies, we learn divrei kabalah. We learn things like oneg, mimtzo cheftzecha, and daber davar on Shabbos, from Yeshayahu; or we learn things like moshav leitzim from Tehilim; etc.
My question is this: Before those things were written in Tanach. were they practiced? For example, before Yeshayahu came, was there a requirement for oneg, just that it wasn't written down?