Airplanes that took off in Ukraine or want to land there are no longer welcome over the territory of Belarus. This is the answer to Ukraine's airspace closure, because the country has joined the ban of the European Union.
In response to the not entirely voluntary landing of a Boeing 737-800 from Buzz at Minsk Airport, the European Union decided that airlines from Belarus are undesirable in the Union territory. An extensive ban was pronounced. Ukraine has joined this. Belarus is now countering and locking out too.