No engineer has ever sued because of constructive post-interview feedback

For example, https://www.eeoc.gov/eeoc/newsroom/release/10-26-17.cfm

When you do a google search for lawsuits related to hiring discrimination, you are far far more likely to find the cases that became publicized because they have merit. Journalists and other organizations don't tend to write headlines along the lines of "Google got sued by candidate for baseless claim of discrimination, but suit was quickly tossed out". It doesn't mean these lawsuits don't happen.

In fact, it's a huge PITA to defend youself when someone accuses you of discrimination. Imagine an engineer who isn't very bright, but doesn't think that of themselves. So you tell them that they didn't do well, but they disagree. And then they sue on grounds of discrimination. Now, you have to prove that you don't discriminate, which can be pretty hard to do.

A company didn't think you are a good fit. That's it.

The criteria they used to evaluate that is totally different than the criteria other companies will use.

What is the point of providing feedback for a data point that does not correlate with other interviews?

I’ve seen this and done this as a hiring manager at multiple companies.

If you are honest, clear, and direct, and your interview (key point) does a good job quantifying the candidate’s performance, and their expectations are well set before the interview, there’s little risk of a reputation hit.

I’ve had folks push back or disagree, but it’s pretty rare and I’ve never been worried about someone dragging the company into a spat.

If they want to take that on board, it's up to them and it might well be helpful moving forwards.

I will personally give people feedback if its tangible and reasonably objective but if someone just feels wrong, I think it is easier to say they were not a good fit and leave it at that.