It's fine in some scenarios, but it runs the risk of sounding like you're joking and being offensive. Definitely don't use that language in a poor & violent neighbourhood! But in a posh restaurant it's generally fine. Of course, if you're unsure of the person's gender, don't say Sir or Ma'am, as that would be presumptuous and potentially mistaken.

That's a really great question! You will hear people saying sir, miss, maam, etc. still as a polite gesture, formal, esp when talking to a stranger in the States or trying to get someone's attention. Currently in my part of the States (to my knowledge) we do not have a popular gender neutral term that fits into this category. I tend to not use those terms due to this as I can possibly misgender someone . i will if I know someone, my family is southern and it is very common to call people older than me by Miss /Ms. or Mr , unless they tell me otherwise . What I normally do when I am at a restaurant with servers, i will ask their name, I will refer to them by their name . When in doubt, always ask :)

"Madam" is rarely used now. "Ma'am" is somewhat common, especially in the southern part of the United States. "Sir" is very common, in shops or restaurants, and in some other formal settings.