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Setting up a deactivation callback is often unnecessary because most of the things you might think you'd need to unset on deactivation should actually happen when a user uninstalls the plugin. Deactivation is not an appropriate time to delete plugin options, remove database tables, delete user content, or perform any other highly destructive actions.

Users and even WordPress itself may deactivate a plugin for any number of reasons. You don't want your plugin users to lose any important data when deactivating only to reactivate and find their settings or content gone. Use with extreme caution.