The events of Game of Thrones season 8 will likely play out differently in George R.R. Martin's A Song of Ice and Fire novels, but it may still have confirmed a book readers' Targaryen fan theory. Season 8 was a Targaryen-heavy season, with a focus given to Daenerys' descent into 'madness' and aim to take the Iron Throne for herself, which in part was inspired by the reveal that Jon Snow himself is really Aegon Targaryen.
Of course, Aegon Targaryen isn't just Jon Snow's name, but also his secret half-brother, the firstborn son of Rhaegar Targaryen and Elia Martell, who was supposedly killed by the Mountain as a baby during the Sack of King's Landing at the end of Robert's Rebellion. In Game of Thrones, that is what happened, but GRRM's books present a different take on things, and one where Aegon might have somehow survived by being switched out for another, random child and then smuggled to safety outside of Westeros.
The character, first introduced in A Dance with Dragons, goes by the name Young Griff in order to hide his true identity, which is allegedly Aegon IV Targaryen, thus making him the rightful heir to the Iron Throne. However, one of the most popular fan theories is that Young Griff isn't really Aegon Targaryen at all, but instead a pretender, and most likely from the long-lost lineage of House Blackfyre, a cadet branch of House Targaryen who frequently rebelled against them over the years, and were believed to have gone extinct in 260AC (well over a century before the events of Game of Thrones). Other versions posit he is Illyio Mopatis' son, or just a boy of the right age with the right look, but what matters most is the common belief that he is a fake, typically dubbed 'fAegon Targaryen'. This is something Game of Thrones season 8 hinted at.
This plotline was infamously cut from the series, as part of their streamlining that also saw stories such as the Dornish conspiracy trimmed right down, but its impact can be felt in Game of Thrones season 8. Daenerys' attack on King's Landing is likely to be against Young Griff in the books, and so too will her eventual downward spiral by motivated (and least partially) by having another claimant to the crown. In the latter instance, at least, we can see how Jon Snow took more than just Aegon Targaryen's name, but also his role as the person to push forward Daenerys' Mad Queen arc, albeit with some differences.
It's in Jon Snow's story, really, that we can see Game of Thrones suggesting Young Griff really is a fake. If he was real, then it'd be harder to cut him from the show anyway, but the way season 8 hinges so much on Jon and Dany confirms just how much this is their story. The books are shaping up to go that way in the end too, but that means there isn't room for yet another Targaryen. Varys' support of Jon lends further credence to this, since he's supporting Aegon in the books. The Golden Company's involvement, too, can be seen as a dismissal of Young Griff: they work for him in the books (which is one of the many reasons he's suspected to be a Blackfyre), but here they not only work for Cersei, but are easily wiped out, which plays down their own importance.
Game of Thrones had already cut Young Griff, but in season 8 it gave us a much clearer picture of how he's going to be used, both as a means for Daenerys' turn and her destruction of King's Landing, and did nothing in that regard to suggest he really is a Targaryen. Since Dany beat Cersei in Game of Thrones, where she'll defeat Aegon in the books, and that what really matters is Jon and Daenerys, it's another major sign that the fAegon Targaryen theory is correct.