Wow. You know how all NFP defenders talk about the 1880 and 1853 decisions supposedly allowing the rhythm method ( though they don't, they just overlook it? )
In 1886, it turns out, the Sacred Penitentiary said that confessors should actually interrogate penitents about their sɛҳuąƖ practices, reversing former decisions that stuck close to the lenient confessional style of St. Alphonsus. If they were found practicing any form of birth control, they were committing mortal sin. However, the author of this book does not say if the rhythm method was classified under birth control.
This decision is in the Decisionae Sanctae Sedis. I can't find it online.
Of course, since the modern heretics are disposed towards NFP, they only regurgitate the decisions that help their cause. They try to make it seem like there is a clean, straight line leading to NFP. But pastoral practices changed over time and what was tolerated as a lesser evil in 1880 may have been forbidden in 1886. Either way, the rhythm method was never permitted -- just overlooked. No decision EVER said it was without sin, not until Pius XII.