The issue has gone to the margin

blueblooded65 can we say "it has gone to the margin"?

That is odd.

blueblooded65Or what can we say instead?

Perhaps you mean that 'it has been marginalized'.

blueblooded65 When an issue is not that important as before, can we say "it has gone to the margin"? Or what can we say instead?

I have never heard that. You can put something on the back burner when it is no longer quite so urgent.

It has become less important.
It's of diminished importance.
It's not critical anymore.

If you suggest postponing discussion: Let's take it off the table for now. / Let's set it aside for now.

I think "set aside" is a good choice.
For example can I say: "Nowadays, religion has been set aside by youngsters."

These days, youngsters are becoming more secular.

That's right. But I mean to say that very sentence if possible.

"Nowadays, religion has been dismissed by youngsters."

I have never heard "gone to the margin."

Some options for your sentence might be:

"Nowadays, religion is given marginal importance by youngsters."
"Nowadays, religion has been relegated to the sidelines by youngsters."