"In the Roman rite, the prayer to St Michael is still said (as part of the Leonine prayers) after Low Mass:"
In the Tridentine Rite, yes. In the NO Mass, not that I've ever heard or seen.
"In the NO Mass, not that I've ever heard or seen."
I guess the people who wrote the NO didn't want to do anything that would keep Satan away.
We say it after every Mass. NO or TLM.
My family says it after Mass no matter where we are.