No it's not and you're presenting a false dichotomy. They made the decision who to fire (to "cut once"). The question is email vs. having managers meet with their impacted direct reports which can definitely and is regularly done inside of one day.
Telling people "you may or may not still have a job, you'll get an email within the next 72 hours" is ridiculous. So what happens if it's Monday morning and I didn't get an email? Am I still employed? What if there was a glitch? What if I show up and I'm escorted out?
This screams of mismanagement.
To do this you would have to already have finalized this list of people to let go. Specifying that list beyond rough department targets to actual individuals probably requires involvement of many managers and might be what happened in those 72 hours, but I'd love to hear from someone with actual experience. Could larger organizations actually prepare that without leaking the layoff anyway?
You still have a job.
>Am I still employed?
Yes.
>What if there was a glitch?
This seems rare, and it seems like people would double check to ensure this doesn't happen (and if it did, to re-send an email).
>What if I show up and I'm escorted out?
Sounds awkward at worst, but now you know.
Play that out a bit. Is the notification a scheduled slack message or email for those terminated? Or do you want each employee being let go to be notified in a face-to-face conversation with their manager and where the details of financial arrangements, healthcare continuity, and other aspects are prepared and with a chance for the employee to ask questions, and for HR and leadership to assess if they appear in a condition to commute home and offer other transport or acute support arrangements if needed?
I think the latter is more appropriate, but a series of those things takes time to execute [longer if managers or layers of managers are being eliminated], and the time between when the first such meeting ends and the last such meeting begins, people are left wondering.