If you search up "Parallax Software Corporation" in the Illinois Business Entity Search (https://apps.ilsos.gov/businessentitysearch/) and view the file history, you will see that there was an application for reinstatement processed earlier this year, meaning that Parallax as a legal entity still exists. The company is pretty much just Mike Kulas and Matt Toschlog, who are listed as officers in the entry.
The references I provided mentioned that Parallax (Kulas and Toschlog) owns the copyright to the Descent games; under a legal agreement, Interplay cannot continue to sell those games without paying royalties to Parallax. However, Interplay owns the Descent trademark and publishing rights (which is why Volition was unable to develop Descent 4 after parting ways with Interplay, and instead developed Red Faction - https://www.youtube.com/watch?v=EmIOps1mEVk).
It's easy to assume that Parallax just ceased to exist after the formation of Volition and Outrage. After all, Volition was born out of what staff members remained at Parallax after the split. But it is a little more complicated than that.