If I create a custom GitHub action and want to share it only within an Organization (all repos within that organization should be able to use it), is it possible by hosting it in an Internal Repository? If not, what is the alternative? I don’t want to put it in a Public Repo / Marketplace.
You can today create organization workflows that others can consume, by using organization templates, with the caveat that they can be made private only if the organization is part of an enterprise account: Sharing workflows with your organization - GitHub Docs
While there is the enterprise requirement, this should help organizations who need to maintain privacy in their org workflows.