OK. It’s not (currently) “law” as such–not in the strictest sense.
It’s a “guiding document,” an interpretation of certain principles involved in the Mass and how to deal with certain problems.
Here’s what I mean. Someone asks a question “what happens if the priest gets sick and has to leave in the middle of Mass?” (gee, sound familiar?)
The Church doesn’t just ask that question as if it were being asked for the very first time. We’ve already known the answer for 400 years now, so there’s no ned to come up with a new answer. We’ve known the right way to handle it for 400 years, so unless something comes along that would change the answer (like something more specific to the Missal used at the time, and which is different in today’s Missal), the answer itself will be the same as it’s always been.
I cannot say to what extend De Defictibus is still taught in seminaries; and that’s going to vary from one place to another and from one professor to another. What I can say though is that in principle it’s still being taught. The questions we see here on CAF are often those asked in the course of seminary training. After all, every priest needs to know what he is supposed to do if he gets sick during Mass, or if he accidentally uses water&water instead of wine&water at the offertory, or any of several other issues addressed in the document. The issues themselves are timeless and the responses don’t change. Seminarians today might never have heard of the document itself, but unless something is very lacking in their formation, they do learn the principles of it. Seminarians love to ask questions like “what do I do if a fly falls into the chalice?” If you think that happens a lot on CAF, just try being a fly on the wall in seminary liturgy courses.
I don’t know why it’s no longer printed in the Roman Missal. Personally I think it should be, or at least an updated version of it should be printed there. I think it would be useful. The GIRM does tell us what to do, but information on what to do if something goes wrong is useful information.