As with the term Symphony, the term requiem has exapnded it's definition beyond simply the Catholic service for the dead. Even within the more specific definition using the Catholic text and order of service, not all Requie (or is that requiescit) are created equal. Mozarts is the probably the most famous and uses the complete text, however, Faure, which is perhaps the most popular excludes much of the text and could rightly be called a mini requiem.
On the broader interpretation, the German Reguiem is accepted as a requiem because of the title and intent but it is not even intended to be paert of the Catholic tradition and does not use the Latin text at all. Personally, I find it dreadfully dull, but that is neither here nor there as far as this thread is concerned.
There are other dramatic, figurative, uses of the term Requiem, that intend n ot the Catholic service but a rumination on mortality and the possibility of forgiveness and redemption. I would site only "Requiem for a Heavyweight" as one example.
There are many who say that Shostakovich 14 is his version of a Requiem because it also deals with the same issues. Being an atheist he finds no solace in religion.
So, in the broader sense of the term, I would posit that Parsifal is as close as we will get to a Wagnerian Requiem.
It is overtly religious.
The overarching theme is a return to oneness with God, entailing restitution, forgiveness and finally, redemption. The story of Kundry is particularly focused on this. Being as she was cursed for laughing at Jesus on the Via Dolorosa, she spends the rest of her prolonged existance trying to make up for her sin. She does it through the one word that she utters in the final act "Dienen" which is to serve. Parsifal redeems the Grail nights and Amfortas by the restitution of the Spear. Through these acts Parsifal brings oneness between God and Man as shown by the light of the Grail and the Dove at the end.
Because the text and story of Parsifal are Wagners own, the symbolism and intents cannot be fobbed off on a librettisist. Wagner is working his way through his feelings, as an older, sick, man. Looking death in the face and asking what does it mean and is there more?