When championship Riven came into the stores she didn't become available again until 4 years later during "Worlds". Is that going to be the same way with Championship Zed? Another 4 years till we see this skin in stores?
Yes, that is how all Championship (read: Worlds-release) skins will be handled.
EDIT: Misread OP. All Championship skins will return every year for Worlds. Riven's might not, but with her separate 2016 version, they might remove the exclusivity (read: 1 in 4 years) thing and make it every year as well. So it is likely Zed's will not be super exclusive and will return every year as well.
