Two trans am sites have this to
Y88 : 1978
This was called "Gold Edition" and was only produced in 1978, it had Gold paint finish with brown stripes and Tan interior, they could have t tops or not and still labeled Y88
And this is from on this site.
Special Edition Trans Am which had the Y88 designation, but just like in 1976, it did not matter if the car had t-tops or not, they were all the same Y88 code.
So that to me would show that there were y88s made without t tops