The parts of Donbas that didn't (successfully) separate from Ukraine could vote, the parts that did successfully separate and become the Donbas republics could not vote for Zelensky. I may have misunderstood you as there are a lot of ways to draw a political map of Ukraine. We have the actual separatist regions that turned into the Donbas republics, the regions where there was a separatist uprising that was crushed (Mariupol and Lysychansk most famously), regions with significant separatist sentiment but without large uprisings, separatist and Russia sympathetic regions , Russian speaking regions , and finally Crimea.