What is the WordPress query var system and how do you add custom query vars without breaking WordPress rewrite rules?

Question

WordPress Fundamental — Hard

What is the WordPress query var system and how do you add custom query vars without breaking WordPress rewrite rules?

Accepted Answer

WordPress query vars are recognized by an allowlist. To add custom query vars without breaking rewrite rules, use the query_vars filter to add them and access via get_query_var(). Remember to flush rewrite rules on plugin activation/deactivation with flush_rewrite_rules().