Is s`en aller informal?

Salut James,

Yes! Possibly.

The expression is taking a sense of "I'm off, bye -- See you" ___"Je m'en vais"

But the lesson points out also a rather dismissal sense of sending someone on their way.

Bonne journée

Jim 

Yes, s'en aller is used frequently in informal speech, just as Jim surmises. You can can also use it as a rather rude way to tell somebody off: va-t-en!