5) The maniple is an article of liturgical dress used in the celebration of the extraordinary form of the Holy Mass of the Roman Rite. It fell into disuse in the years of the post-conciliar reform, even though it was never abrogated.The implication is that some people were slovenly about it and it just stopped being worn and that the less casual should wear it, especially, as it was never abrogated.
Now, I am not sure weight one should give to an online document but it is from a Vatican office, and it is on its official website, which has increasing been used to promulgate law and teaching.
Despite its ancient antecedents it was simply dropped from the General Instruction of the Paul VI Missal in its list of vestments for Mass. If it was never abrogated it should be worn.