This idiom or expression refers to:

A period when certain restrictions don’t apply.

A. You know everybody’s killing rats?
B. I know, it’s open season on them!
A. Speaking of open season, are Republicans still in trouble?
B. Yeah, everybody is still blaming them.

Also, related:
Fair game.

This idiom or expression refers to:

The object of an attack, especially when certain restrictions do not apply.

Celebrities’ personal lives seem to be fair game these days.
The candidates’ financial activities are fair game and open to questions and criticism.

Also, related:
Open season.

This idiom or expression has to do with:

Finding out who is more of a coward in a group.
A game to determine who gets scared more easily.