Did plainchant ever completely die out?

[XB-70 Valkyrie]

Obviously this newfangled thing called polyphony became all the rage in the 13th and 14th centuries. When did this lead to the extinction of new compositions in the old plainchant style--or did it? What are the most recently composed plainchants? What are the most recently composed ones that have been accepted into church (any church) canon? Do any recent composers have any interest in this style unaccompanied by any harmony or instruments?

How can plainchant be adapted to modern music--or can it? Obviously, many modern composers have been inspired by this style--Duruflé, Part, Martin, Pizzetti, etc., but I mean real, bona-fide, unaccompanied plainchant.

[Karl Henning]

In terms of usage, it has certainly never died out.  Plainchant has been in consistent use in liturgical worship not only in monasteries, but as an integral part of Eastern Orthodox services.  There have also probably always been Catholic and Anglican parishes which have continued to include monophonic chant in their liturgical cycle.

I am not alone (and certainly nothing like the first ) to write fresh plainchant, specifically an original psalm tone which is one of the elements in my unaccompanied choral setting of the St John Passion.

[BasilValentine]

It is not certain that chant was ever composed in the sense you mean. The preponderance of evidence suggests it was an oral, improvised tradition before the need to codify and regularize the musical liturgy drove the effort to transcribe and thus fix for all time then current improvised performances. In a chant seminar I took as a graduate student, one of my colleagues, for his term project, learned to improvise certain kinds of chant using formulae gleaned from studying examples in the Liber Usualis. Chant was still sung at Catholic masses in my youth.

More direct answer about dates: The repertoire was pretty much fixed about 1,100 years ago, although some specialized things like sequences were added later.

[Monsieur Croche]

Thought to be antique, but most certainly written very recently:

https://www.youtube.com/v/BwkH1SAwphY

[Karl Henning]

I would even say it glows.