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.
My husband does this with his phone (not a number he uses for work). My parents do this as well and I can’t figure out if it’s due to lack of tech skills or not wanting to deal with voicemails (I think it’s a combination). I had surgery a couple years ago and had to give the hospital all three numbers and then my brother an hour away as backup since he’s the only one besides me with functional voicemail.

This. I’m surprised more people dont havent mentioned this, but this has always been my back to work routine – and sometimes if vacations overlapped, I’d find a summary email of “While you were away, X,Y,Z happened, I covered A, but you might want to check on B and C” which was always very helpful.
There’s a department at my workplace where this is common practice too. My old department worked with clients in similar ways, and I was half expecting we’d also be required to do it, but luckily that never happened. Further proof that, at this (generally progressive, flexible-working) company, your actual work-life balance heavily depends who manages you.