I have often wondered about this document & whether or not (or to what extent) it is still in effect…especially on the issue of pianos at Mass.
However, for dioceses of the United States, the GIRM has a special provision which does allow pianos in U.S. churches… The thing is that I’m in Ireland, and (I should check for sure one of these days) I am not aware of any such provision in the GIRM adaptations for Ireland - so pianos could still actually be banned from Irish churches even though many parishes have purchased digital pianos in recent years.
It is clear that many later documents, such as Sacrosanctum Concilium and Musicam Sacram, only reiterated several points made by Pope St. Pius X - such as Gregorian chant having pride of place, emphasis on the people being able to sing some of the chants, the organ is to be esteemed higher than other instruments (apart from the human voice)…all of these points, and more, were made by the Council Fathers. Of course some points were abrogated by Vatican II, such as the question of who should be admitted to chant in the choir - Vatican II opened the possibility of women singing in the choir, for example…though, I suppose that those who use the 1962 liturgical books might still be bound to Tra le Sollecitudini to some extent on this issue.
So it seems to me that there are parts of the document which could be said to be still in effect (such as the issue of pianos in some countries) and are to be found in more recent documents, but some points, as I said earlier, have been clearly “overruled” by later documents… That would be my reading of it anyway, and, like Ocarm, I am certainly happy to be corrected by one of our experts!