The plan B is to send it again when they’re back though. I think there are a lot of situations where I emailed them and someone else from the start and someone else answered, or I found someone else in the 2 months span until they’re back, or did it myself, or it’s too late to help…. So it does help them avoid following up on things that don’t need following up on.
Or, worse, when someone has left the organization and the organization hasn’t bothered to put up an OOO, so I’m just emailing a blackhole until I call or someone finally checks that inbox. I never fail to set up my OoO reply, and yet most of my external contacts don’t get them. Let’s say I work for LlamaCombs, with an name(@)llamacombs.com address, and this is a company who has two clients AlpacaBrush and VicunaShampoo. I work primarily with the second, and their internal directory lists my contact info as name(@)vicunashampoo.com. It works because any e-mail sent to the second address is auto-forwarded to the first. Except it messes up OoO replies big time. Because the auto-reply is sent to my own alternate address, not to the original sender, and I have no way to change that.

Carnegie Mellon's U.S. campuses observe 11 official holidays. The university is closed on these days, and all non-essential personnel are not expected to report to work. Regular, full-time staff members may also take up to three floating holidays per calendar year.
I say this as someone who used to have a chronic problem keeping up with my personal voicemails. But I got voicemail transcription set up so I can read them now, because just ignoring important phone calls has consequences. I can’t imagine trying to just duck them in a professional job where I had a phone number, and therefore an expectation that people can call me!