They’re not de-canonized: the Church can’t de-canonize a saint! They’re just not on the calendar anymore.
The Church can in fact, de-canonize certain saints. How?
Because of the number of names, some of popular saints removed from the calendar, the tendency is to affirm that the Church never de-canonised anyone, their names are just not celebrated universally, etc.
This is true to an extent. Canonizations of the formal sort are in fact, infallible, and most theologians also believed the same about beatifications. But this applies to the formal and solemn definitions/permissions of the Pope. Canonizations have also occurred due to popular acclaim before the process was standardized, and each case would have to be examined to determine the extent to whether infallibility, if any existed in the case, would be present.
Likewise for the beatifications, and here perhaps is where Simon, William, and other similar saints come in. The type of ‘beatification’ granted was a simple confirmation of a public cult i.e. acknowledgment that a public cult was in force - known as the aequipollent beatification - and almost all the theologians held that ‘confirmation of cult’ is by no means infallible and is subject to the rulings of the Church. Since the Congregation of Rites suppressed the public cult of Simon of Trent, it can in fact be taken as a revocation of sorts of the ‘beatification’ and together with the expurgation of his name from the liturgical books, means that unlike many other saints, he is not entitled in any way to , for example, have a Mass said in his honour, not even from the Common of Saints.