The first person of authority in the Church to profess the belief in Invincible Ignorance was Pope Pius IX. Before this happened baptism of desire was understood to only mean catechumens can be saved without water baptism, after this happened the false teaching of I.I. was incorporated into baptism of desire (for catechumens).
I'm sure most people know that at the beginning of Pius IX's pontificate he was liberal and his actions and teachings, not necessarily sinful, but were un-traditional.
He first propagated I.I. in Singulari Quadam in1854, then Singulari Quidem in 1856, and Quanto Conficiamur Moerore in 1863. This was during the same time when he was doing many liberal things.
Pius IX's theological outlook became very different in 1864 and onwards, read
http://fatherfeeney.org/other/masonry.html to understand that.
In Vatican 1, seesion 2, 1870, Pius IX took a profession of Faith, at the end of this he states,
"This true catholic faith, outside of which none can be saved, which I
NOW freely profess and truly hold, is what I shall steadfastly maintain and confess, by the help of God, in all its completeness and purity until my dying breath, and I shall do my best to ensure [2] that all others do the same. This is what I, the same Pius, promise, vow and swear. So help me God and these holy gospels of God."
Pius IX is specifically repenting for his belief in I.I., why else would he be saying "Now"...The definition of now is- at the present time or moment...that clearly means he did not formerly believe "outside of which none can be saved" before. I read the entire session 2 of Vatican 1 and no where else is "Now" used when talking about a procession of belief or codification of dogma. After 1863, Pius IX does not ever speak of I.I. nor is it mentioned at all in Vatican 1.
Also in the procession of faith, Pius IX says, "I confess one baptism for the remission of Sins."
Also, "I profess also that there are seven sacraments of the new law, truly and properly so called, instituted by our lord Jesus Christ and
necessary for salvation, though each person need not receive them all.
I think it is clear that Pius IX does not believe in I.I. after 1863 and he repudiates it in his Procession of Faith.
I look forward to what spam LoT puts on this thread.