What I did say was that it was not de Fide until the definition. Assent to it was not required.
Currently it is not decided whether Mary was assumed before or after her death. Clearly, if we grant the Assumption, one or the other must be true. But it has net been declared one way or the other. The Church may (and presumably does) discuss it.
Before Nicea it was okay to argue Arianism. Now it's not. Before Chalcedon one could argue monophysitism or monothelitism. I was just made aware of a monothelite movement arising among some Catholics. Because of Chalcedon, as I read their stuff, I can see right away that whatever the relationship between God's will and man's IS, THEIR version is certainly erroneous.
There still is this notion that the Church runs the flock with a tight rein. That's just not true. It's only when an issue comes to a kind of boiling point that definitions are made.
And once they're made they're settled (in theory at least) and we can move on.
Definitions do not make something true. We may be jerks, even Big jerks, but not THAT big.
One more stab at it. If you wanted to become a Catholic in 1953 you would not have to give assent to the idea that Mary was immaculately conceived. But now, when we instruct people who want to come into full communion, we try to lay out both the big Marian Dogmata, to explain them, to say why we think they are worthy of belief. And we say if you cannot assent to this, you really shouldn't be coming into full communion.
Here I keep getting told by Romanists that their church is the one that the "truth once delivered" was delivered to but yet apparently not all the truth?
So if someone wants to join the Romanist Church today they have to assent not only to "Mary mother of God" but to "Mary mother of God with additional premises"?
What great heresy was the Romanist Church fighting that made them have to declare that Mary was sinless?