After Ministeria Quaedem, who may be a Subdeacon for a Missa Solemnis?

In a [Missa Solemnis](https://en.wikipedia.org/wiki/Solemn_Mass) there is a celebrant, a deacon, and a subdeacon. However, the [minor order](https://en.wikipedia.org/wiki/Minor_orders) of subdeacon was suppressed by Pope Paul VI in the *motu proprio* *Ministeria Quadem* in 1972. Nevertheless, since the reform of the Mass in 1969, the Tridentine Mass has been celebrated at different times, still having a subdeacon at all Solemn Masses. So, after the minor orders were abolished, how is it that these groups can still have a subdeacon for Solemn Masses?