Link to documentation of the D-Bus APIs used by Flatpak apps
Since Flatpak arrived, the API surface of GNOME changed significantly. Rather than specific C library APIs being important, the boundry is now mostly specific D-Bus objects which are exposed into the sandbox.
This info relevant when app developers need to know which version of GNOME their app can run on; and for system maintainers to gate changes to relevant D-Bus APIs to avoid breaking existing Flatpak apps.
I don't propose writing reference documentation here, just a note that the D-Bus APIs are the main point of contact between Flatpak apps and the OS, link to Flatpak's own documentation on this, and some links to the reference manuals for each component that exposes D-Bus APIs to Flatpak apps.