After upgrading the portal to 4.4, the API Explorer cannot be seen as a menu option within Publish. The documentation doesn't have any release notes posted so it cannot be confirmed whether this is intended or not.
API Developer Portal 4.4
In Portal 4.4, the API Explorer is only accessible through the API Portal/Ingress tenant (apim.<domainame>). It was replaced with the "SPEC" (SwaggerUI) interface. So if you are using an external tenant, test and explore APIs using the Swagger UI. The major difference is that SwaggerUI (Spec) does not have the ability to generate sample code for multiple languages the way the API Explorer did. It only generates a curl call.
Please see our doc on Testing and Explore API's.