When a business rule "AFTER" set is used to integrate with external applications with WORKFLOW, which can be sending outbound SOAP/REST calls to a third-party application, below log is posted:
com.glide.sys.OutboundRequestValidator$LicensingError: No IntegrationHub plugin is available, external calls from the Flow Designer requires IntegrationHub subscription
As it is mentioned in KB article KB0783502 , For Flow Designer ; user needs to activate IntegrationHub plugin to use the corresponding flow action in Flow Designer.
This is also valid for Workflow not only for Flow Designer.
This is working as expected since outbound integrations that are triggered via flows or workflows should require IntegrationHub.
The fact that using "ASYNC" in business rule can work for now but it is not definite and will not work in the future.
The preferred solution is to activate integrationHub starter which gives 1 million free transactions / year.