I do think it's important that table manners are taught. And I'm surprised that many of the things mentioned above are seen as particularly strict, as they were just normal in our household. Rules like waiting until everyone has finished for example shows consideration for others who may eat more slowly and who don't want to be left at the table to eat alone. Personally I used to let my very young DC leave the table if adults were chatting for a long time, but the rule was you had to ask to leave.
Not leaving the table during dinner is surely just basic manners , so as not to disturb everyone else and disrupt the host's service. In France, it seems like hardly anyone visits the lav during an entire evening, they all have steel bladders, or ladies hardly drink, but I would be interested to know the official rule about this!
I do know it is good manners in France to always keep your hands in view above the table (so the opposite of UK) and you rarely eat just with a fork, you virtually always use a knife as well. And it's fine to mop up a sauce with bread although that would be frowned upon in lofty circles in UK.
And many high born Belgians (and Germans perhaps?) would not thank you for being disapproving about having beer glasses on the table, even in smart restaurants, although the glasses would probably be stemmed and balloon shaped, depending on the brew. I have seen people in Spain use a small piece of bread to push food on to a fork and to drop olive stones on to the floor - collectively I mean - so perhaps that is permitted there? I'm not sure tbh!
As long as you are considerate of others (elbows in, mouth shut while chewing, not hunched over your plate (so you can see and talk to others and they can see you) passing serving dishes along and being aware of what's going on, and helping yourself last, contributing to the conversation, asking people about themselves and not just talking about yourself all of the time, not making any eating noises that are considered rude in that culture, and thanking your host and complimenting them on the food served, I think you can't go far wrong tbh!