In the first leak video I can find on his channel, he reveals that a source is telling him Isabelle, Ken, and Incineroar are coming to the game. At this point Vergeben had already leaked Isabelle, Ken, and a gen 7 rep, but did not confirm it was Incineroar until after Shifty's vid. This same video also confirms that Isabelle was the very next fighter that would be revealed (which was true), and that Rathalos, Dracula, and Marx would show up as bosses in the then unknown blurred out mode (Spirits Mode).
https://www.youtube.com/watch?v=xdYhLFiJIsY
His next leak video claims that Box Theory was true, which was correct.
https://www.youtube.com/watch?v=K2wX71R-G1I
This next video claims Ultimate DLC would go on for two years, and in all honesty it looks like this will come to pass. This at least would've made us aware that DLC would continue after the fighter pass.
https://www.youtube.com/watch?v=VcmAVb8sGz0&t=291s
The next one was a leak of the Smash Direct in November. He revealed a second source which was recommended by his first source in all the previous videos. This source claimed that the Direct would be coming November 2nd. It actually turned out to be the first, but it was pretty damn close.
https://www.youtube.com/watch?v=lfcEwxiNdGk&t=1s
This is the last leak video before the Ryu vid where he deconfirms the Grinch leak, and confirms Piranha Plant of all characters (PP was leaked by the second source).
https://www.youtube.com/watch?v=Q8Wq3uuPDyg
Finally we have the Ryu videos. Obviously there was controversy surrounding these videos because he claimed Ryu, in no uncertain terms, would be the 4th DLC fighter. This turned out not to be the case. The second video, which discusses the SNK leak, claims that neither of his sources knew of SNK being in the game (indeed, almost no one seemed to), but reiterated that they had heard Ryu would certainly be in the fighter pass.
https://www.youtube.com/watch?v=JcwSKSy-nIY&t=6s
https://www.youtube.com/watch?v=zWP1fHUK0kA&t=1s
So most of his claims have been proven true. One has yet to be confirmed (DLC lasting 2 years), and 2 were proven to be wrong (although the direct date was very close). The Ryu leak still has the possibility to be partially right if he turns out to be the last fighter on the FP.
As for this possibly being mixed up information with Ryu being added to Warriors Orochi 4 Ultimate, IAmShifty told PapaGenos that this was possible, but unlikely because he claims his sources come directly from Nintendo. He says it doesn't make sense for his sources to confuse the Ryu leak with a game that Nintendo has no involvement with. Here is a link to a video that displays this conversation starting at 13:51:
https://www.youtube.com/watch?v=yUHvd0KGTCg&t=888s
There's definitely a precedent for this. Most people still trust Vergeben even though we've yet to see Minecraft content and the fact that he said the first DLC character would be a SE rep. His source knew of the SE character, but not precisely when they would arrive. We continue to trust him because he has gotten so much correct in the past, and the same is true for IAmShifty. I think it is plainly clear at this point that Shifty has legitimate sources as Vergeben does.