If "required" means necessary or obligatory:
* Optional: This implies something is not necessary, but is available if desired.
* Unnecessary: This means something is not needed.
* Voluntary: This implies a choice to participate or not.
* Discretionary: This suggests something is left to individual judgment.
If "required" means demanded or requested:
* Offered: This means something is presented as a possibility, not a demand.
* Suggested: This implies a recommendation, not a requirement.
* Optional: This means something is not mandatory, but available if wanted.
* Unrequested: This means something was not asked for or desired.
The best opposite will depend on the specific situation.