This is a breaking change and isn't deprecation, deprecation keeps the behavior and just marks it as deprecated

I would be better to keep the new dialog and and just tag the method as deprecated.

I can't just deprecate the method. If the dialogs are going to be merged, either FileDialog needs this functionality, or EditorFileDialog needs it removed. With the way I plan to do the merging, leaving side menu only in EditorFileDialog will require some hacks.

Then that should be part of the discussion, but regardless the descriptions and the naming should be updated to reflect that the method was removed rather than deprecated

This was designed for parity with Blender:

As for only being used by glTF... well, so far. There are other places (I don't remember where off hand) in the editor dialogs that add checkboxes to the bottom of the dialog, which IMO should be migrated to the side menu. Regardless, we would need to do something with those checkboxes with native dialogs.

I would prefer to keep this feature, and perhaps we need to dynamically make it behave like it looks in this PR when native dialogs are enabled, perhaps by making the file dialog hide most of its UI and contain a native dialog showing button instead of the save button. So essentially, FileDialog would "contain" a native file dialog, and go directly to it when using the native dialog AND there is no side menu, but otherwise it would show its own UI.

Regardless, we would need to do something with those checkboxes with native dialogs.

Checkboxes and option boxes are supported by native dialogs, but adding fully custom elements like this is completely impossible (it's not a limitation of the current implementation, it will never work).

I would prefer to keep this feature, and perhaps we need to dynamically make it behave like it looks in this PR when native dialogs are enabled, perhaps by making the file dialog hide most of its UI and contain a native dialog showing button instead of the save button.

This is the current state. If native dialog is used, and side menu is set, it pops up as a separate dialog before file selector is shown.

I changed the config dialog to appear after selecting file:

It's more consistent with other software.

Thanks!