Content Pipeline - Platform Differences

I’m unsure how the chosen platform in the pipeline tool affects the output file and whether the output is compatible with other platforms or not. If, for instance, I loaded levels via a text file through the content pipeline and set the target platform to Windows, would DesktopGL platforms be able to use that file?

Generally yes (depends on your project setup), but as there is no content being processed you can add such text files as a normal embedded ressource as well - if you prefer.

you normally have 1 content file per project/platform anyway, so it shouldn’t really matter. While having a global content is technically possible, it does not work with current MG because you have to change a line in the .targets file to fix the parameters for mgcb.exe

What if instead of text files it was a Tiled map processed via MonoGame.Extended’s TiledMapImporter? Aside from a tileset texture, the map it processes should work regardless of platform. Would it be possible for people running a WindowsDX version of the game to import a map from someone who created it on a DesktopGL project (Ex. running on Linux or macOS)?

It depends on the type of content. Some type of content might need to be processed differently depending on the platform. For example shaders. If you run an OpenGL version of MonoGame, then the shaders will be compiled for OpenGL. If you run a DirectX version of MonoGame, then the shaders will be compiled for DirectX.