Manners are always necessary, IMO, like decent clean clothing. Etiquette is not vital, but may be appropriate based on context, like say your church outfit (or red carpet gown lol). I think attitudes to etiquette are probably similar to one's general philosophy on how important or useful it is to be able to "fit in" in the surrounding environment, and how much of your self confidence depends on your being in tone or inversely, a rebel.
Personally, I trend conformist, and I like to know, and like my son to know, proper table etiquette. That said, it has been about 3 years that we have started taking our son to "upscale" restaurants, on special occasions. DH & I have always treated ourselves to a fine-dining experience for our wedding aniversary. And when DS was younger, we got a baby sitter for him and went alone. Then 3 years ago, he said he wanted to come celebrate our anniversary too - after all he was a member of the family too and deserved to participate . So, we didn't downgrade our experience, we upgraded his - he had already been eating out at more casual restaurants, pizzerias etc.
Thank heavens in this country there is no such thing or even discussion of "child-free environments". Children are welcome everywhere, and everywhere I have been I have seen children able to behave themselves appropriately. Maybe it depends that it is so normal for children and adults to socialise in many occasions and contexts, both formal and informal. Maybe it depends that this is a very child-friendly culture, so my idea of "appropriate" would be some other cultures idea of running wild. I don't know.
My child can pick a plate for himself at a buffet gracefully. He can use a napkin. He can use silverware. He knows what glass is for water and what for white or red wine or spumante (though he only uses the one, of course). He knows to stay seated reasonably composed and keep his elbows off the table and speak in a conversational tone. If a meal progresses too long, he might get bored or restless. Then he may leave his seat and go outside if possible, or otherwise he sometimes comes and sits in my lap and we play hand games like scissor rock paper, or he gets his dad's phone and plays some game on that. And that is perfectly child appropriate, non disruptive behaviour in any social context. Actually it is the same sort of behaviour he has to maintain at home (with a few pieces of silverware less and no buffet!). It doesn't have to be a big deal.