There exists a letter of John VIII to Constantinople IV (879-880) which explicitly rejects the Filioque. Written presumably after the council as it is added afterwards to the council’s Acts in Mansi, John VIII asserts:
we again declare to you that not only do we thus recite it [the Creed without the Filioque], but even condemn those who, in their folly, have had the audacity to act otherwise from the beginning, as violators of the divine word, and falsifiers of the doctrine of Christ, of the Apostles, and of the Fathers, who have transmitted the Creed to us through the councils; we declare that their portion is that of Judas. (Source; for another, highly variant rendering see source)
It otherwise expresses caution in imposing the council’s ban on those presently using the Filioque, asserting that patience should be exercised on the question. Such a detail, so obviously diplomatic and non-polemical, screams authenticity.
Nevertheless, historically Western writers have rejected it, feeding into the conspiracy theory that John VIII never agreed to the council’s rejection of the Filioque. Schaff refers to this letter as “pretended.” (Source) Dvornik observes it is “not impossible” for the letter to be authentic (The Photian Schism, 198), but he clearly considers it either forged entirely or highly altered. Other, knowledgeable Roman Catholics assert the letter is “altered” and “a clear forgery.” Contrarily, Guettee confidently asserts that “this letter was published from Western manuscripts,” (Source) a claim I cannot verify.
Is it authentic or not? To answer the question, I propose we look elsewhere–John VIII’s Letter 11, To Svatopluk in Moravia, identified as Industriae tuae.
The background behind this letter is given to us by Dvornik:
[Saint] Methodius, accused by the Frankish clergy of heretical teaching [against the Filioque], was summoned by Pope John VIII to appear in Rome to justify himself. With him came also Wiching, the leader of the Frankish clergy and the chief accuser. We learn from the letter addressed to Svatopluk in June or July 879, that the complaint against Methodius was voiced in Rome by another of Svatopluk’s councillors, John of Venice, and that the accusation was endorsed by Svatopluk himself. In letters to Svatopluk and to Methodius the pope expressed his astonishment at this accusation…However, another missive to Svatopluk, sent in June 880 and starting with the words Industriae tuae [i.e. the letter from Pope John VIII to Svatopluk] provides evidence that Methodius justified himself by explaining to a local synod, presided over by Pope John VIII, what he and his disciples believed. The pope, after commending Svatopluk for his fidelity to St. Peter, stressed in the letter that Methodius professed the same Creed as Rome. This was natural, because in Rome, as in Byzantium, the Nicaean Creed was still recited without the addition Filioque, of which the popes did not approve. The pope, however, stated in the letter that Methodius was found orthodox even in other articles of faith, including the doctrine on the Procession of the Spirit. (Byzantine Missions Among the Slavs, 164-165)
As follows is a translation of Pope John VIII’s aforementioned letter:
Accordingly, we questioned this Methodius, your venerable archbishop, in the presence of our brother bishops, whether he adheres to the creed of faith [fidei symbolum] in the orthodox way and during the sacred liturgical rites sings as is held by the Holy Roman Church [i.e. without the Filioque] and as was announced and established by holy six universal councils of holy fathers according to the evangelical authority of our Lord Christ. He thus declared that he believes and sings [the Psalms] according to the evangelical and apostolic teaching, as the Holy Roman Church teaches and as was established by the fathers. Moreover, we, having learned that he is orthodox and useful in all ecclesiastical teachings and matters send him back to you again to govern God’s church. (The Slavic Letters of St Jerome, 23-24)
From the preceding, one can see there is an independent witness that John VIII implicitly rejected the Filioque. If one continues reading the letter, the Latin available here, John VIII admonishes Moravia to expel “schismatic” missionaries who resist Methodius (whether rejecting his adherence to “canonical tradition,” i.e. the Creed without the Filioque, or rejecting his use of Slavonic as opposed to Latin is not clear). In any event, considering John VIII endorses Methodius on the question of the Creed and in the same breath pushes for ecclesiastical as well as legal discipline against dissenters increases the probability that John VIII did write a letter to Saint Photius affirming his rejection of the Filioque. John VIII supported church discipline on dissenters concerning this question, aligning him with the Horos of Constantinople IV.
Orthodox Christian Theology 
Just when you think scholars have gone over the details of the filioque with a fine tooth comb you bring up a really interesting point. Is the letter of Pope John VII genuine? I wonder what the textual history is?
But you do bring up good evidence. The Papacy hadn’t adopted the filioque in the late ninth century so the Popes would naturally support Methodios over aggressive Frankish bishops.
Depends. The Popes were under the thumb of the Eastern Franks. There are nuances to the politics involved where Frankish Pneumatology worked against Frankish foreign policy and it was useful to subvert Frankish ecclesiastical influence and replace it with a directly Roman one. Methodius was made both a priest and bishop in rome if i remember right.
These finer details of history aside, the real crux of the issue remains truth in advertising. It remains difficult for Roman Catholics to admit that the insertion of the Filioque as a latter-day innovation, no matter the justification or rationale, because they know it was adopted in the west centuries after the promulgation of the Nicean-Constantinopolitan Creed. They’d be better of calling it the Nicean-Constantinopolitan-Toledo Creed, or Carolingan Revised Nicean-Constantinopolitan Creed, so something along these lines that accurately reflect what it is. That they don’t, and never have, tells me they know they are shaky ground but to save face and the Papal construct built around it, they can’t admit it. Yet.
And here’s how you’ll know. I’ll bet dollars to donuts that if some Latin Rite priest were to recite during the Mass the Creed without the Filioque,or a Latin Rite bishop were to publish a missalette with no Filioque printed in it, nobody in the pews would notice or care, and would easily follow right along and say the Creed without the Filioque, with no protest or problem.