Hi @thackeray ,
Adobe Experience Manager (AEM) is a comprehensive content management solution for building websites, mobile apps, and forms. AEM makes it easy to manage your marketing content and assets.
To answer your question "Is AEM used widely as a developer portal?" - Not really, there is no such specific things from the AEM applicable to use it. Additionally, It depends on the org requirements.
However, Adobe Experience Manager (AEM) is an enterprise content management system that optimises the authoring, management, and delivery of content and digital media. AEM allows you to create unique digital experiences and manage fully context-specific messages that scale across products, services, organizations, and countries.
There is no OOTB functionality which supports Open API Specification rather content type to it.
Hope that helps!
Regards,
Santosh