Is there a way to find out what's included in a plan (projects, boards, etc.) via the server REST API?
The portfolio API does not seem to include this information, and I haven't been able to find it from the other side either (coming from the project/board data).
It sounds like you're after the plan's "Issue Sources". This isn't currently part of the Server API.
Please feel free to create a JAC suggestion and vote on it. We use these suggestions to know what features to add to our roadmap.
If you're after the issues themselves then you can use the Jira JQL search api.
The Portfolio for Jira Team