Speaking about absences, I thought it was weird that they didn't mention the role of the other Riders in Skyrider at all, considering how it's a fairly big part of the show, with about 20 episodes with past Riders, including the one where he gets his upgrade and the final arc.
I finally got through all of the Urban Legend Quizzes from Fourze (I hadn't seen the ones after Stronger until now). Why didn't they make episodes for Black and Black RX rather than doing a two-part final exam? Is it because part of RX technically took place in the Heisei era?
Although we usually look at the "Showa" period as an uniform classification, there are really quite a few divisions there, like the Legendary 7 (the uninterrupted original run up to Stronger). The first 10 Riders often was seen as a cut off point too, since they all had Hirayama Tooru as the main producer and there's another break of a few years before Black and Black RX. So, due to that, several Showa-related works seem to use ZX as the stopping point, like the Kamen Rider Spirits manga and the Kamen Rider Eve/Gaia novel.
It also doesn't help that all remaining ones (Black, RX and the 90s movies) are mostly standalone, with no further appearances of the past Riders. Even Black RX, although featuring them in the final 7 episodes, mishandled and minimized that appearance so much that it's not surprising that their appearance there is usually overlooked with ZX seen as the end of their story, rather than Black RX.