Are we talking about momentarily checking if there's an important interruption and continuing with the conversation? If so, I'm confused as to why you possibly think it's unacceptable and rude.

I can see how this could be considered rude in a very formal setting, or when matters of high importance are discussed and uninterruptible focus is a must. But for most casual situations, I perceive glancing at incoming notification as normal. Assuming this happens reasonably infrequently, of course - that is, a glance in a while, not constant checking every other minute.