these girls do all sorts of things to tease their pussy-- nude colored thongs, pull panties up in their pussy lips, grow hair/landing strip, put baby oil around their pussy, put water or lube on their panties to mimic a wet pussy-- all sorts of things.
They have gotten creative and learned from each other. If a girl like Mariah goes this long without showing anything, it's doubtful they ever will. If they do show anything, they will quickly back off. OF has evolved to the point of the girls having all these tricks.
In fact, these non-nude girls probably make more money not getting nude. Their simps fall into two categories: 1) those still holding out for them to get nude, or 2) those with some weird parasocial relationship where they DON'T want them to get nude. Many girls like this literally LOSE subs if they show too much.