She was dating Rami Malek and he broke up with her (don't know more details than that) at the end of the second season. She didn't want to be around him after that, so she asked Sam Esmail to write her out of the show.

This was incredibly unprofessional, in my opinion. Mr. Robot was Sam's Magnum Opus, and the final plot revolved heavily around Portia, so to write her out completely ruined everything. That's why the final season feels like freestyle, amateur jazz, at open mic night. (in the boring part of town)

Most people seem to support her decision. I'm not one of them. I'm guessing most casting directors agree with me, though, so that's why you haven't seen her lately.