Is 2y = 8 a linear function

Yes it is. because if you give a number to (y) you can find the outcome.

I hope you serve.

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