The Yoshi series is out-dated in terms of games, but it still gets references and the like in mario spin-offs. This includes every mario spin-off of which shy guy is in. Also shy guy is generally placed near or around Yoshi in screen-shots or official videos, I also found this on super mario wiki:
Shy Guys' first chronological and true Mario appearance is in Super Mario World 2: Yoshi's Island.
This is interesting since we both know it a Yoshi game, is it not?
Only two things to say:
1) No one cares what the characters first chronological appearance was. The point is, the character originated within the Mario series. Guess what? The same game was also the first appearance for Mario
chronologically. It was the first game in the entire series chronologically.
2) ^ That doesn't even matter due to a point I continue to bring up, but you continue to ignore. I'll quote myself so I don't have to type it again:
Shy Guy's trophy being placed in the Mario series rather than the Yoshi series. Pure, undeniable evidence. This shows that Sakurai and most likely his development team view Shy Guy as a Mario character, not a a "Yoshi character". This proves that, as far as we know, Shy Guy would be a Mario character if he was playable.
Don't even try to counter this with your regular "Sakurai could have changed his mind" stuff.
Until you come up with a logical argument for this, you have no hope of ever being right. If you can't come up with a logical argument, you need to just admit you are wrong. To help, I'll show you a couple arguments you've already tried, but already failed:
1) "Sakurai could have changed his mind." ~ We have no reason to believe this. We've explained enough why this argument sucks.
2) "That evidence is out-dated." ~ Yoshi's Island is also out-dated. But that doesn't even matter, because no matter how outdated it is, it's all we've got. You've got nothing. Find another more recent piece of evidence to counter this, or don't use this argument.