As an interviewer, candidates occasionally ask me "How did I do? Where could I improve?" when I ask them "Do you have any questions for me?" at the end of the interview.

I usually say (and I happen to be in a country that is not very litigious) "While I can't tell you how you did in this interview -- we have to go through HR's process first -- I could offer you some general advice based on my experience", and would give some positive suggestions based on what I've observed. E.g. "try to read up more on how technology X works under the hood" or "when faced with a complex problem, it's okay to take time to think it through or ask questions", or "your communication skills are good, but you can be even better if you lead with the answer and have the lengthy explanation follow the answer" etc.

I'm wondering whether this is a good practice?

What exactly was the policy that caused this? In my (luckily very limited) job searching experience, the salary is something that gets negotiated. I ask for something on the high side, they offer something on the low side. Repeat until you end somewhere reasonable. The only thing I can imagine causing someone to not be hired on salary without them knowing the reason is if there's no negotiation.

That's a cultural thing in the company I work for (or maybe broader in Germany, dunno), where some people think that if they start to negotiate, they might lowball candidates, and then they leave after half a year or so.

We try to hire more long term (we aim for 5+ years retention), dunno if that's related, or if there is any evidence to back this procedure.

I find it a bit weird myself, which is why as a candidate I'd like to know about it :-)

As an employee I found a nice hack around that. The employer made an offer and I made a counter offer, the employer said he couldn't meet it and I explicitly told him it was negotiable. He thought about it for a couple days and found a way to hack his system to give me more and I got something around the middle of our offers. We both ended up happy in the end, I'm still there.

Right.

It's better to offer the candidate what the company is prepared to offer, both in terms of position and compensation. Let the candidate decline. At least some candidates will like the company enough to try working their way back up.

Though I'm sure acceptance rate is in some KPI, so there could be some bureaucratic nonsense involved.

I find it odd to reject someone that way, unless the expectation and range are way off.

Normally, the good interviews begin with some kind of high-level salary discussion with HR or the recruiter. Typically, the recruiter (or HR rep) casually asks what I make in my current job. In one case, I accepted an interview where I'd get a small paycut. (Some things are worth taking a lower salary.)