Meg,
my mistake, I forgot to post the reference; its from the Denzinger, Pius X, "Praestantia Scriptura" Dz-2114
https://patristica.net/denzinger/#n2100
Thanks for providing a source. It comes from a Motu Proprio called, "Praestantia Scriptura" by Pope Pius X. Unfortunately, the Popes aren't bound, as far as I know, by Motu Proprios of other popes. But still, it may apply to others in the hierarchy, but how much force does a Motu Proprio really have? After all, it's obvious that all of the Modernists in the Church at that time didn't leave the Church after fearing an automatic excommunication by the above Motu Proprio. They probably ignored it.
I don't understand why Pope Pius X didn't call a Council in order to address the heresy of Modernism. That's what Councils are supposed to be for - to address error, and clarify and promote truth. It would have been more binding of course if a Council had condemned the heresy of Modernism. Though the Modernists may still have ignored it. Why God allowed the Crisis to occur isn't really clear.