Should I contact a company whose interview offer I ignored to apologize?

With my experience in hiring, at a medium-sized company. Interviews were covered by so many people and we read/sent out follow-ups to so many people we would never have remembered one not answered. The only thing that would remember is our hiring software that remembered people by email address.

I once did this (one of the top-5 embarrassing things I've done in my life). I actually ghosted a job offer many years ago because I felt uncomfortable about it. A few years later, my company at the time actually merged with the company I ghosted.

I took the opportunity to apologize in person to the manager in question. He basically just shrugged, didn't care/remember.

In light of my experience, I recommend that you not bother contacting the company to apologize for the oversight.

If it was essentially your oversight and a whole year has passed, I think I would just let sleeping dogs lie and hope they've forgotten your name by now.

It's old fashioned to imagine an employer (or his secretary) writing out a letter by hand, to a candidate they might realistically foresee retaining in a job for life, and holding a grudge for non-response.

Everyone knows emails today are flying around like a blizzard, and it would be extremely petty for the employer to have inferred (and also recorded, and recalled at the time of your later application) anything adverse from non-response to one interview request, months or years prior.

I think you've answered this yourself because you've listed only positive reasons to contact them. Try something short, pleasant, and light on excuses: "Hi. Sorry I missed your email while I was preparing for employment at another company. I hope you found a great candidate."