Found out I might have unknowingly cheated after term completion One of my classes had a midterm scheduled right after we were all sent home due to Covid-19. The professor said that for this midterm, we were allowed to use notes, both his and our own, browse the internet, and talk with peers during the exam. While this seems abnormal, we, as a class, checked multiple times and he confirmed this policy every time. Thus, during the exam, about 2/3 of the class (~10 people) got in one video call and were talking during the exam.

For the final, the professor simply said that the rules would be the same as the midterm listed above. To our discredit, no one double checked what the professor meant and simply went with it. Of course, the same ~10 people got on the video call again and did the final. It was not an easy final, despite us collaborating and looking at notes, leading me to suspect nothing wrong. The professor also had not posted a note saying collaboration was allowed on the exam, although this was the same as the previous midterm, so I thought he had just overlooked this. Looking back, I see the fallacy in this, but it did not seem important at the time.

However, recently I was talking to an acquaintance from the class, one of the 10 in the video call, and I found out that he had indeed checked with the professor about peer collaboration on the final, which the professor explicitly disallowed (this was during office hours). This acquaintance said he did not tell this to us as he needed us helping him on the final to pass the class. 

I'm conflicted on what to do now, as it has been over 3 weeks since the end of semester (4 weeks since the final in question) and most of the class was in on the video call. If anything, all our answers were EXTREMELY similar (most were exact copies) to the point that it would be obvious we were collaborating. On one hand given the above, it seems unnecessary to dig this up. On the other hand, I'm not sure that I'm not reasoning my way through having cheated. Please advise.
Assuming that the no-collaboration rule was really not communicated to you, then you didn't cheat.  You followed the rules as you reasonably understood them based on the information you were given.  There's no such thing as "unknowingly  cheating"; it requires intent.

However, there is a substantial risk that the professor may think you have cheated, if he mistakenly thinks he made it clear to the entire class that collaboration was not allowed.  So it is in your best interest to clear it up before any formal charges are considered.  Just explain what you've said here: you were told that the rules would be "the same" as the midterm (if you have this in writing, better yet), and you later found out that other students were told something different.

The fact that the term has ended means nothing.  It is entirely possible, in all academic systems I know of, to impose a punishment for cheating even after the term is over, perhaps months or years later.   And it is not too unusual that investigations can't be completed before grades are due, in which case the professor may assign the grade based on an assumption of no cheating, but it can be changed retroactively if his investigation confirms wrongdoing.

So even if you don't want to "dig it up", the professor still can, and you will convey more honesty if you get out ahead of it.  You have a good defense, but it won't look as good if you have to say "well, I found out afterwards there might be a problem, but I didn't say anything because I hoped you wouldn't notice".

Note that your acquaintance definitely did cheat, if he's telling the truth about what the professor told him: he'd been clearly informed that collaboration was not allowed and he did it anyway.  Moreover, he did it in a way that placed innocent students at risk of being suspected and prosecuted for cheating.  Many people would feel that you also have an obligation to report this to the professor.