The REST API that the Flex API depends on didn't come along until 9.3+. In theory you could still write a Flex app to use the SOAP services exposed by ArcGIS Server 9.2 and below. But (a) you wouldn't get the benefits of the nice ArcGIS Flex object model, and (b) in practice I've found that the Flex SOAP tools do a very poor job of building code from ArcGIS SOAP.