While we Texans might find it polite or friendly, in the corporate world I’ve been told to avoid using it in emails.
The singular polite "y'all" I'm referring to is generally used when the other party is not known. F.ex. I regularly, though certainly not always, hear clerks, waiters, or other service industry workers using "y'all" singularly when asking "what can I get y'all to eat" or "y'all need anything else" when speaking to exactly one person, and I use it that way myself.
I think this is the formal singular "y'all" that people keep referring to. It can sometimes be short for "you and your family," which is why you would see it in greetings and goodbyes e.g. "How are y'all doing today" or "I hope to see y'all again soon."
For signs of formality, it has the indirection of not addressing the person directly (like "your grace" "your honor" or "your mercy"), and is usually used as an stylized expression of concern. To my ear, "How was y'all's holidays?" sounds really professional, like how your lawyer would start a meeting.