The following is copied from the Indiana University website:

“The bulk of Indiana University’s Russian Military Topographic Map Collection is made up of the Soviet Red Army topographic maps, which were produced for defense and economic planning. This collection came to Indiana University from the duplicate map room of the Library of Congress Map Collection in the early 1990s. These maps cover not only parts of Russia and Eastern Europe, but extend as far north as Scandinavia, as far west as Germany and the Netherlands, and as far south as Iran.

“An interactive index map of the collection is located here: https://iu.maps.arcgis.com/apps/webappviewer/index.html?id=098c42997ca441029b69f0597ff92ea6

“For more information, visit the Cyrillic Maps Collection.”

There are very few maps of Central Asia, but coverage of Russia, Belarus, Ukraine, the Baltics, Poland, and eastern Germany is pretty good. The maps of former Soviet states are uncopyrighted, but odds are good that coverage outside the USSR was taken in violation of local copyrights and so may not be used in OSM.