There are examples of this (with various punctuation and spelling choices) dating back to 2011.
October 26, 2011
But Mr. Klees is insisting that his move to seek the Speaker’s chair is not so game-changey at all, telling reporters on Tuesday:
Sun May 15 2011
In just those few lines about the Time Lord called the Corsair, we get a lot of game-changey stuff, too
FEBRUARY 5, 2011
We here at Vegan Kicks love to find out about new companies and brands who sorta come out of nowhere and do big, game changey thangs.
I would not be surprised if all of these were completely independent coinages. Ad hoc formation of adjectives using the suffix -y is common.
Now, the company has released a more traditional Halloween promo video for the series, but with a perfectly Stranger Thingsy twist
Just LOVE your Jack Russell whiskers and your dachshundy ears!
@ddale8 Very Louis the fourteenthy