When I was a child back in the 18th century, if, for example, you were out walking with a friend and you ran into someone you knew, you would always introduce your friend. It would be considered the height of rudeness to leave someone standing there not being able to “properly” participate in the conversation as they had not been introduced. There was also even a protocol for how you introduced them, which I always managed to get wrong. It was something like you’d introduce older people to younger people, and so on.
So I recently ran into a friend - she was with somebody else but she didn’t introduce this person, who then stood awkwardly waiting for us to finish. I feel like this is incredibly rude, but I acknowledge that it’s not the first time it has happened in the past few years, and maybe I’m just stuck in the 18th century.