The St. Michael prayer and the three Hail Mary's are just plain not included in the Order of the Mass.
It's not a question of being mandated. Pope Paul VI just never included them in the Novus Ordo.
One of the "rules" is - if there is no rule, it doesn't exist.
Unfortunately, that is only followed in cases of Orthodoxy, not inventions.
I get your point - the Leonine prayers have been quietly forgotten by the liberals - as if Pope Leo XIII's mandate never existed; similar to how the modernists who've taken over our chanceries quietly forget about Pope St Pius V's "Quo Primum" as if it never existed.
As you probably know, abrogation of the traditional Latin Mass was never mandated either.
In other words, priests of the Roman rite should still be allowed to use the Leonine prayers and traditional Latin Mass without restriction.
Isn't it strange that such open-minded "laissez-faire" liberals, who permit practically everything else, can't tolerate the traditional Latin Mass - or even just the Leonine prayers?
Such otherwise "tolerant", mild-mannered (even limp-wristed) individuals seem to a fly into a rage at the mere mention of anything "traditional".
I wonder if satan has anything to do with the status quo?