Is 2y= 8 a linear function

Yes its. Its only one posibility to not have linear function. If we have for example x=2, the graph will be vertical line. Then we get infinitely many values of y for the same x. As we know in function for every x can be only one y value.
2y=8      /:2
y=4
Its simple for of eq of the linie y=ax+b where a=0 and b=4