I propose that when a file is opened in Blender 2.8 using a render engine currently not available, that it provides a warning to the user, probably in a pop up. The first thought that came to mind was a warning for all files that had Blender Internal set as the active renderer, however, support could be added to everything not in Blender such as Radeon Pro Render, LuxRender or AppleSeed. The proposed idea is:
If not Cycles, EEVEE, Workbench, + addon render engines (the developers of the engine would have to implement support for this), display a warning.
There was a feature for this in Blender 2.79b, but it’s one of this top/bottom bar messages that are usually ignored and quickly disappear. The feature appears to be missing in Blender 2.8.
Another thought is possibly warning users if a material for an object is setup in a render engine currently not available, however I’m sure there’ll be difficulties handing this.