selfoss offers a RESTful JSON API for accessing or changing all selfoss data. You can use this API in your custom selfoss client app or any other program or plug-in.
The API endpoinds are specified using OpenAPI description format, which you will find in the selfoss repository. Or you can view it in a more human-readable format on SwaggerHub. There are also many tools that allow generating boilerplate code from the API description.
Since selfoss 2.19, the API follows semantic versioning distinct from selfoss itself to allow tracking API changes in development snapshots. Below, you can see which selfoss release offers which API version: