“Shatner only found out in [a specific year], from [a specific person].”
Unsupported
Correction
He says he doesn't remember the moment, and he was discussing it publicly by early 2008.
In 2021 Shatner said: "I don't remember the exact moment." Fans on TrekBBS were already describing his comments on Late Night with Conan O'Brien and on his own website in March 2008, and he talked about trick-or-treating in the mask in 2009. Any article giving a date or a source for his discovery is guessing.
What people actually said
“I don't remember the exact moment but I thought 'Is that a joke? Are they kidding?'”