Since we don’t know the name of their captain, I’m going to refer to him as “Rox”. So, somehow this all-powerful Rox crew was taken on by the Navy. We do not know what happened of their battle, but it is known that Garp earned the title of the Hero of Marines because of his confrontation with Rox. We do not know whether he was killed or not. I don’t think he died as then Navy would have been boasting about it form a long time.
So, in my opinion, this Rox went into hiding. Now, let’s talk a bit about Kaido and Big Mom and their connection to Rox pirates. We know that to rule over the seas all alone Rocks needed a crew that would consist of very strong members. And in chapter 907 we got hints that Kaido and Big Mom were a part of this huge crew. As Big Mom tells Kaido that he owes her from a long time ago and the fact that Rocks were revealed just after that. Further, Hina clearly mentioned that Rox Included Kaido and Big Mom, and their Captain was a strong guy.