Imagine this from the perspective of an interviewer. Alice is interviewing Eve for a job at FooCorp. Eve rocks the interview. Alice is happy and gives Eve a high-five and some glowing feedback.
Then Bob interviews with Alice. Bob goes belly up. Never before has an interviewee done so poorly. Given the established precedent of providing feedback, what is Alice to do? We have a precedent in play. Would you want your employees telling candidates that they did a rubbish job? If she declines to tell Bob how he did, he'll interpret it as a negative. If she tells Bob that everything was dandy, it will set Bob's expectations.
The only reasonable choice is to bar your employees from providing feedback. Any other course of action is unfair to them.
This implies that feedback can't be provided throughout the interview. Reduce feedback loops, etc etc.
In some of the 'worst' interviews I've conducted, it would become clear with 45 minutes to go that the candidate was a pass for technical reasons. After exploring for other strengths beyond the expected skillset, I focus the rest of these sessions on the really important things the candidate should know before their next interview with some other company. This is easy to present as standard interview questions, but with more explanation and context worked in to setup and 'next step' in multiparty questions. With no explicit "you've failed" ever stated, you have an opportunity to maintain a positive conversation, do the candidate a service and a favor by educating on what they need to learn next, and if you're really lucky discover that the candidate is a fast and eager learner who would be a great addition to the team despite current deficiencies.
Worst case, it's clear to the candidate where they're lacking without ever needing to state is as such. And who doesn't learn a little bit themselves every time they teach?
Then Bob interviews with Alice. Bob goes belly up. Never before has an interviewee done so poorly. Given the established precedent of providing feedback, what is Alice to do? We have a precedent in play. Would you want your employees telling candidates that they did a rubbish job? If she declines to tell Bob how he did, he'll interpret it as a negative. If she tells Bob that everything was dandy, it will set Bob's expectations.
The only reasonable choice is to bar your employees from providing feedback. Any other course of action is unfair to them.