How do I stop a Jira automation rule assigning work to someone who is out?
Test one of the app's absence fields in a JQL condition and branch on it. assignee.outofoffice-today = 1 matches work items whose assignee is away today, so the rule can skip the work item or assign the coverer instead.
Related pages
Jira Automation Rules for Extended Out of Office Scenarios - where this is covered in full.
Automations and Workflow - more questions on this topic.