I've never asked about LinkedIn in an interview. However, I do sometimes look up candidates prior to interviewing them, and admit to being slightly curious if I can't find them, although would not let it count against them. Conversely, I have also seen a few candidates look me up on LinkedIn prior to their interview, which I generally take as positive sign, because it shows they are trying to prepare.

For quite some time I was trying to get people to look at my actual resume instead of its shadow on linkedin and I couldn't get people to do it because they would google my name and the first link would be on linkedin so they would never bother to look beyond it. As soon as I closed my linkedin account all of a sudden people started looking at my actual resume and my code on github. I don't regret closing my account and forcing people to do some legwork to find my qualifications instead of doing the lazy thing and stalking on linkedin.

It's not unheard of for employers to look you up on every social media site as a quick "background check." They don't have to ask you about it in an interview for it to be relevant.

I've definitely been asked by potential clients if they can look at my LinkedIn profile to get an idea of my background and the sort of companies and projects I've worked on in the past (and I've even had clients ask that about other members of my team too).

Not at an interview, but I've certainly been asked about it in the informal social conversations that often are the precursors to eventual formal interviews, and people often seem skeptical when I say I don't have one (much in the same ways some people might react skeptically if told that someone doesn't have a github account).

Not having a github profile is fine and should raise no eyebrows same way not having a linkedin profile is fine. Resumes used to be a thing before linkedin and if someone says they don't have a resume then maybe that's an issue but otherwise it's business as usual. Similarly if a programmer has no side projects then that might or might not be a type of person you're trying to hire but github in no way is a prerequisite for having side projects which is what actually matters and is an indicator of how much a person invests in programming as a skill.