[ck425]

I don't read regards as fundamentally rude but I do read it as very cold and formal, which can come off in as rude or passive aggressive depending on the context.

I wouldn't bat an eye if a letter from my bank or a formal HR communication signed off with "Regards". But if a colleague from another team or a senior leader ended an email with "Regards" I would find that rude.

That said I've spent the last two years working in the public sector and have come to learn that the standards for communication and presentation are very different here. I'd still consider it rude from a colleague who worked in my directorate (it's a tech directorate) but if it was from someone outside my directorate I'd give them the benefit of the doubt. Though in those cases I also tend to be aggressively informal/casual in my responses.