The story
From pan- + post-Soviet (“relating to former members of the Soviet Union”), as an extension of pan-Soviet (“spanning the Soviet Union”) by analogy.
Etymology from Wiktionary, CC BY-SA 4.0.
pan-post-Soviet
Today it means: Spanning all or most of the region which was once part of the Soviet Union, especially in reference to the post-Soviet countries (countries which were once part of the Soviet Union). Full definition →