With the recent news of returning Kamen Riders, we do have news of one Kamen Rider that is not going to be in the film. Hiro Mizushima who played Kamen Rider Kabuto on Instagram answered why he is not appearing in Hesei Generations Forever.
Hiro was replying to a comment wanting to clear the air of any or all misunderstandings about him not appearing in the film. The answer was very simple, they never approached him to be in the film. Hiro wishes success for the film. He even stated that he appreciated working with the producers, directors, and staff of his series. Hiro sees Kamen Rider Kabuto as the starting point of his career.