It is the complete opposite from my view, whereas "most of the people" would think carrying two phones to be quite ridiculous.

Some people do here, but perhaps I'm not brave enough

I'd be worried about mixing up business and personal chats and photo galleries. Like if I need to take a picture with my work phone it's usually of something work related.

Or forwarding the wrong meme on a business chat, or attaching the wrong file in a family/friend message could land me in trouble.

Quite unusual. It's also harder to purchase devices with 2 (physical) SIM slots (a popular feature in Asia)

The two-phone solution seems to be giving way to BYOD. I haven’t carried a second device in years.

Our telco salespeople must be really happy for selling two phones to each of us here.

How does it work in NDA'd work environments? Like when company's information can't leave company's devices? Is BYOD accepted in those cases? Like with MDM managed phone or workspace separation?

Microsoft 365 (e.g. the Outlook and Teams apps) has a bunch of certifications which seem to mollify some CSOs. In high security environments I would imagine work-issued phones are no better.