I don't expect to be contacted again after a screening conversation or a simple application, but if I am interviewed in-person by multiple people I consider it common courtesy to provide at least a from letter "No". This is becoming very uncommon these days for the various reasons given, as well as the fact that there is little downside to being rude - it's not like the candidate can do anything to the interviewing company.
However, in my own hiring I always send a thank you for the candidates time, because I'm old, and manners, etc. but also I consider it the long game. You never know when you will run into someone again and in what context, and the extra effort to email a "thanks for interviewing with us" is tiny compared to potential upside/downside.
> it's not like the candidate can do anything to the interviewing company
It's true to a point. However, when you ghost someone, you've effectively as a company burned your bridges with someone, all for want of a simple and not at all burdensome small amount of courtesy. Even if you're not interviewing them in person, a simple notification that the application will not be considered further means they aren't left hanging on pointlessly.
If you want to hire them in the future, they might just blow you off.
If they pass on their bad experience to others, your reputation has suffered and you'll make it harder to recruit others in their social circle.
You might be immune to the effects of rude behaviour in the short term, but it will come back to bite to some degree later. As an example, after some really rather abrupt telephone interviews with Google, and a really rude on-site interviewer, I told them never to contact me ever again. I have no intention of considering them again in the future.
I think your point about considering the long game is spot on. I had the dilemma of interviewing with two companies and getting two good offers. The company I ended up rejecting were really nice, made a really great effort to sell themselves, and I left with nothing but positive feelings. Nearly a year later, they would still love to have me on their team. Should I need to in the future, I'm fairly certain I could give them a call and start work the next day, all because they did a really good job of building a good relationship.
Phone screen from HR? Maybe not. Though I'd prefer a simple form email "sorry, not interested". As noted elsewhere, build it into the HRIS - as soon as the applicants status is flipped to "No", automatically generate the email.
Phone call with hiring manager or team members? I expect a rejection of some sort.
However, in my own hiring I always send a thank you for the candidates time, because I'm old, and manners, etc. but also I consider it the long game. You never know when you will run into someone again and in what context, and the extra effort to email a "thanks for interviewing with us" is tiny compared to potential upside/downside.