That’s a great question. The API is semi-public, in that it’s not publicly documented and available for public use, BUT its documentation has been provided to other Foundry developers (it’s actually a pinned post in the Foundry discord’s developers specific chat channel), and it is being used by the Foundry Hub project as well, as well as by those shields.io badges.
It is subject to change, though I will try to keep it backward compatible as much as possible, or contact those I know are using it (hence why I say it’s semi public) if I make any major changes.
Note that I’m referring specifically here to the Bazaar API which I’ve made available to non-authenticated requests. The Forge API is much more extensive and the other parts of it have not been shared for the most part and remain private. Other than the public (non-authenticated) Bazaar API, the only other instance of the API that I have documented is the one explained here : How to control your games using custom API requests and reset your worlds periodically
installs field represents the percentage of games in which the package has been installed. The reason for using percentages instead of actual number is simply as a way not to leak internal Forge data relating to number of users, games and such (note that a package could be installed by a user but not enabled and Story Teller/World Builder users with the Game Manager would have it count as a single install regardless of the number of games they create which has access to the package).
Can I ask what you’re interested in the API for and if there’s some cool project you’re planning on doing with it ?