We log in to the voicemail account and simply activate any of the 4 greetings. With our Cisco VoIP system, I am able to create a Standard, Alternate, Closing and Holiday greeting, however, I can only activate/use the Standard or Alternate Greeting during business hours. I can only activate the Closed or Holiday during specific times.
It is a shame it doesn’t work for people who are not saved as contacts. Hopefully Apple will realize this shortcoming at some point.

Of course the people on my project never use the calendar for work trips. I don’t find out that someone is on travel until I get the OOO autoreply. Weirdly, everyone puts their personal leave on the calendar and includes the reason, especially doctor visits. I don’t need to know why you are absent.
Yeah, announcing you were going to delete emails unread and expecting the sender to resend when you return would NEVER fly in my office. I’d get executive complaints about that, especially if it went to a client or outside party – if a client can’t reach you, they will reach out to someone else who may not work at your organization and you lose business. I feel like this delete-it-all philosophy would only work for an entirely internal role where timelines are more relaxed, and even then, I feel it’s a bit unprofessional to foist your own catch-up work onto others, especially if they’ve been backfilling for you while you were OOO.