https://infogalactic.com/info/Prime_(liturgy)
> Prime, or the First Hour, is a fixed time of prayer of the traditional Divine Office (Canonical Hours), said at the first hour of daylight (approximately 7:00 a.m.), between the morning Hour of Lauds and the 9 a.m. Hour of Terce. It is part of the Christian liturgies of Eastern Christianity, but in the Latin Rite it was suppressed by the liturgical reforms following the Second Vatican Council.[1]
> As the chronology of Cassian's works has recently been established fairly accurately, the institution of Prime must be placed towards 382.[4]
So from 382-1960s (?) this was observed but the Vatican 2 church broke from this tradition....
Hmm, what do you have in mind going back this far? I've thought jansenism might have been set up as a heresy to then push the opposite heretical reaction, of strict going to loosening the rules. Also there were relaxations of opposition to interest on loans. There seemed to be a weakening over time but no overt embrace of heresy that I could find. But Vatican 2 didn't happen overnight.
I take the sedevacantist view and reject the SSPX / sedeplenist viewpoint, although certainly they're in agreement on many other issues.
And yes the (latin) mass was also suppressed. I have just been posting continually on all the things that were changed to try to get people to question it. To me getting rid of Prime seems in some ways like a pretty big deal like the suppression of mass. It's basically getting rid of the discipline of prayer of the clergy, which would seem to open the Church up to spiritual "attack". To anyone watching it just should look like a giant red flag. All these changes don't look like "genuine" developments but red flags with no real purpose but destruction, or at best being random changes.