Some of the functions of a subdeacon at Mass (holding the paten in the humeral veil) had been already been suppressed during the interim reforms of the mid-sixties. After that, they continued to read the epistle and carry the vessels to and from the altar. To an untrained lay eye of today, a subdeacon at Mass between 1965-1973 would have looked like a second deacon., but one who read the epistle, rather than gospel.
Existing subdeacons in 1972 remained subdeacons until they were either ordained deacons or left holy orders altogether. (We had one man in our parish who was a subdeacon at the time.) Since a cleric’s time in the subdiaconate was usually only a matter of months, anyway, most went on to the diaconate and then no subdeacons were ordained.
It is important to remember that the subdiaconate was a major order of the Church. When a man was ordained as a subdeacon, he was obligated to celibacy and the recitation of the Office, and was styled as “The Rev. Mr. ___.” When one reads texts from the middle ages up until V2, the three orders of the Church are always listed as subdeacon, deacon and priest. Bishop was considered the fullness of the priesthood, rather than a separate order, which is why bishops were conscrated pre-V2, rather than ordained. It was in the (ecumenical) aftermath of the council that one saw the redefinition of orders as bishop, priest and deacon, and the re-establishment of the permanent diaconate. In the centuries prior, it had been debated as to whether subdiaconal ordination was a sacrament or a sacramental, with most theologians apparently leaning toward the latter. While not instituted by Our Lord, nor mentioned in scriptures, the subdiaconate already existed by the year 255, which makes it quite ancient. (Ditto for ember days, and a number of other ancient things that suddenly bit the dust in the same era.)
See newadvent.org/cathen/14320a.htm for a 1911 article on the subject.