Whoops. Got my periods backwards. Actually Showa went until 99. Right?

Showa went until 89, that's when Emperor Hirohito died. (I know this because of Wikipedia.)

89.

Emperor Hirohito died on January 7, 1989 and it marked the ascension of his son, Akihito, to the throne and the beginning of the Heisei era.

Toei kamen rider era ended when creator died. Due to toei classed the three rider movie in 90's as showa. Rider

Then why are the Riders from the 90's considered Showa Riders?

1. Ishimori's involvement
2. Kamen Rider #0 project ("Return to the Origin". Started with Black and ended with J. To be correct EVE/GAIA should have been included as well.)

Because Toei prefers to think of Kuuga as the start of the modern era of Kamen Rider, so Shin, ZO, and J get folded in with the Shows guys.

Toei is kinda odd like that.

The long and short of it is that the actual Showa and Heisei eras doesn't apply to Kamen Rider. They divide them based on how they see fit.
For example, Black RX lasted long enough to qualify as a Heisei series.