If you're writing library code that someone else might use, you don't have much need to understand the features you don't use, unless you have to handle them at the interface. If you're debugging your own code, you really shouldn't have to understand features that you didn't use. (Mostly - see the next paragraph.)

You did say "intentionally". You could wind up using a feature unintentionally, but it's not very common, because most of the new features are either in a new library (which you have to explicitly call), or a new syntax. There are definitely exceptions - I could easily see you using a move constructor without meaning to.

Maintaining someone else's code... yeah. You have to understand whatever they used, whether or not it made any sense for them to use.