If someone is suppressed in your Klaviyo list, that person will still sync to your Facebook custom audience. 

If Person A is suppressed in a Klaviyo list or segment, and you sync this list or segment to a custom audience, Person A will still sync to your Facebook audience. If someone is synced to a custom audience through Klaviyo, and then becomes suppressed in Klaviyo, this person will not be removed from your audience. While individuals may opt-out from email communications, you can still strategically reach these customers or subscribers through Facebook advertising leveraging Klaviyo's integration.