Keep in mind, Reforged is not a different game. People who buy reforged will be able to switch out to HD graphics/lighting/shadows for all pre-exiting wc3 models/icons. The game is the same, so people who do not buy reforged will still get updates every time there are new patches.
The only way Reforged will exclude non-reforged players is if map makers use models that are newly created for Reforged, and do not have a wc3 classic equivalent. So, for example, using all the alternate dreadlord models in reforged for custom maps will not exclude classic players, since they will just see the normal dreadlord unit in game when they play the relevant custom map. The exclusion will only occur when models such as those fancy Stratholme models that just do not have any conversion/equivalent with normal wc3 are used.
In terms of artstyle clashes, I recently played custom maps on reforged beta, and the bird's eye perspective for normal custom maps means the quality of models difference is not really clear from a distance, so people do not need to worry too much about custom models from hiveworkshop not fitting against Reforged quality/artstyle.
