I agree. It really beats me. The organizer did write on the event info page something like "don't share on SNS". While it's a curious coincidence that it's spec

I agree. It really beats me. The organizer did write on the event info page something like "don't share on SNS". While it's a curious coincidence that it's specifically this link that gets deleted, AFAIK there's no way for an organizer to prevent sharing apart from making the event private and requesting attendees in free-text to keep it to themselves. The only thing that I think could potentially be controversial would be that it was a social gathering during the pandemic. The event page itself was and still is up. If it was deemed against FB rules I'd expect it to be reflected in some way on FB, and not just by deleting WhatsApp messages refering it. I really can't give a good reason apart from "some ML model got triggered somehow, which prompted a human somewhere to look at it for a couple of seconds and click the delete button"