This is a complex topic. It needs to be understood what was actually being condemned in the 19th century and what is actually being promoted in the 20th by the Church.
In the 19th century, the Pope intervened on this issue as the Church was concerned with two things being promoted. One was the idea that reason trumped faith and thefore faith had no binding force on conscience. Man was therefore free before God and Church to believe whatever he decided to be most reasonable. This is false and is religious indifferentism, as the true doctrine is man has a duty to embrace and follow the true religion. Man does not have a right before God to commit any sin, or to profess a false religion (cf. CCC 2108).
Second, as a fruit of this erroneous thought, some argued that the state could not put any limits whatsoever on any activity deemed religions. They argued man had a a right to an absolute freedom to do whatever he deemed acceptable. This was called “insanity” because it would lead to the destruction of a well-ordered society.
These things were condemned and still are. The other side of this erroneous coin cropped up with the Communist states that did the opposite–they restricted all religious activity for no good reason.
The Catholic doctrine on religious liberty is more balanced. The Church has always acknowledged that man has a duty to believe and act according to the true religion, but that such faith must be a matter of free and uncoerced determiniation, otherwise it is not true faith. In addition, the state has a duty to protect the common good and defend the natural law. Where false religious activity harms these things, the state may intervene (cf. CCC 2109).
The Catholic doctrine on religious liberty can therefore be summed up like this: man has the right to be free from coercion by the state in religious matters, within the limits of the common good and the objective moral order. To say that this freedom must be absolute and unlimited was condemned in the 19th century and to say that the state could suppress religious activity when the common good and objective moral order was not harmed was condemned in the 20th.
A couple things should be noted: whether or not a state is or should be a confessional state is a different discussion. These principles apply either way. Likewise, a lot of the rhetoric about the Kingship of Christ is misplaced: His Kingship is not violated by the Catholic idea of religious liberty if such a limited liberty is one of His, and therfore His Church’s, doctrines. Such a liberty being granted would therefore respect His Kingship, not violate it. This doctrine also does not mean the state cannot promote the true religion in public, non-coercive ways. Nor does it mean we should not infuse Chrstianity into our laws (cf. CCC 2105).
Second, the Church may coerce the Baptized in religious matters. This was affirmed by the Council of Trent (Canons on Baptism 8 and 14 ) and is found in the current Code of Canon Law (Canons 1311 and 1312; note these canons refer to the Christian faithful, which are defined elsewhere as the baptized cf. canon 204)).
Finally, the Syllabus of Bl. Pius IX is often thrown around rashly by the ignorant. The Pope himself called it raw meat that needed to be cooked. It’s doctrinal value lies in the documents it lists and points to, and that is it. They must be referenced to understand the meaning and wieght of each proposition.
See Bl. John Henry Newman here:
newmanreader.org/works/anglicans/volume2/gladstone/section7.html
See also Newman on the encyclical Quanta Cura (which also addresses the oft-quoted part of Mirari Vos repeated in Quanta Cura):
newmanreader.org/works/anglicans/volume2/gladstone/section6.html
But rather as a way of expressing the fact that they are not being coerced.