8 way demultiplexer hdl not working
You are swapping bits. For example, for b you wrote:

And4Way(a=notSel0, b=notSel1, c=sel[2], d=in, out=b);


whereas it should be

And4Way(a=Sel[0], b=notSel1, c=notSel2, d=in, out=b);


And even better, to avoid further confusion, write the code like this:

And4Way(c=notSel2, b=notSel1, a=Sel[0], d=in, out=b);