I no longer have reforged installed, so I can't check to be sure, but I think there are two ways you can set HD-only.
The first is for your map, which you can set to SD only, SD/HD, or HD only. I assume if you set it to HD only, then 1.32 players using normal/classic graphics are unable to play it.
The second is for individual files, which are either SD or HD. I don't think the map will become unplayable if you have any of these set to HD, but if you do not also import a corresponding SD file, you will have missing models/textures/etc when playing with classic graphics.