Not legal for sure. But there are things that are done informally that are similar. Like at one company where I was a hiring manager we used head hunters heavily. Most head hunter companies are bad (in my experience) but the better ones try to match good candidates with good companies. They have extensive networks with other head hunters and often see the same candidates repeatedly. When they see red flags they cull those candidates or maybe present them to the "bad" companies (the ones they aren't concerned about damaging their relationship with).
Another method is that hiring managers at different companies may have good relationships with each other. Not competitor companies, but adjacent. They may call around and ask about a candidate. You can never say anything negative about a candidate; that's just asking to be sued. I think many (most?) companies have a strict policy that you can only say if a candidate is eligible for rehire or not, nothing else. So you talk in vague generalities, never about the candidate, only about the general hiring process. Something like "Company policy prevents me from giving any recommendation whatsoever, so my advice is that you should always be very careful in your hiring process" versus "Company policy prevents me from giving any recommendation whatsoever, but an effective hiring process can yield great employees." You might argue that a judge would see right through that and say it is obvious which candidate you are recommending and which you are panning. But it's so nonspecific that your lawyer can strongly push back, should it come to that.
Another method is that hiring managers at different companies may have good relationships with each other. Not competitor companies, but adjacent. They may call around and ask about a candidate. You can never say anything negative about a candidate; that's just asking to be sued. I think many (most?) companies have a strict policy that you can only say if a candidate is eligible for rehire or not, nothing else. So you talk in vague generalities, never about the candidate, only about the general hiring process. Something like "Company policy prevents me from giving any recommendation whatsoever, so my advice is that you should always be very careful in your hiring process" versus "Company policy prevents me from giving any recommendation whatsoever, but an effective hiring process can yield great employees." You might argue that a judge would see right through that and say it is obvious which candidate you are recommending and which you are panning. But it's so nonspecific that your lawyer can strongly push back, should it come to that.