You cannot force the entire map to run in a specific configuration as the map maker
Sorry, that's not true. Go into:
Scenario > Map Options > Supported Modes
and change "SD and HD" to just "SD"
You can force classic vs reforged for specific units that are imported into a map, like unit models. You would force either the classic or reforged model by importing that version's model as custom map data (through the import manager) and then setting the relevant unit to use the imported model rather than the original model. You'll want to look into using a CASC viewer to extract WC3 data as a next step to this approach
One interesting effect of this is that you can import a custom model which consists entirely of vanilla textures. The combination of the textures is unique, but the textures themselves change depending on if the viewer has Reforged enabled or not. This has the potential to look unique to your map, while also looking good for both Vanilla and Reforged users. This seems to be the only current way to achieve this result, and so might become the new ideal for model selection