The full source code for the Input System is available on GitHub. This is also where most of the Input System's development happens.
Note: This includes the full source code for the managed/C# part of the system. At this point, the native, platform-specific C++ backends are still closed-source and require a source code license.
Please follow Unity's standard bug reporting guidelines to report any bugs related to the Input System. Don't forget to submit a Project that the developer who picks up your report can use to reproduce the issue. Be sure to mention that the bug is specific to the Input System package in the description, so it gets forwarded to the correct team at Unity.
To ask questions or discuss the Input System, see the dedicated section on Unity's forum. This is also the best place to post feature requests.