Any issues that are specific to Proton/Wine and not present on a Windows build running on Windows directly are not the responsibility of Rewired, nor can these issues even be fixed by Rewired. Rewired is simply a consumer of the input values provided by the input APIs in use. When running under Proton, Rewired knows nothing other than it is being run on Windows and therefore consumes input from the various Windows input APIs.
Rewired’s controller definitions are designed to map elements on a device for each input source they support (Raw Input on Windows for example.) There is no capability of detecting Proton or supporting Proton-specific device mappings for Raw Input, Direct Input, XInput, Windows Gaming Input, or any of the other input sources supported.
Valve is the developer of Proton, so if there are any inconsistencies between Windows and Proton, they would have to fix the issue in Proton.
The Rewired controller definition for the Logitech G29 maps the “Brake” axis to “Rotation Z” on Raw Input and Direct Input. If no value is returned for this axis, then Proton is not sending any value for the Rotation Z axis in the HID input report.