South Africa withdrew from the Commonwealth in 1961, rejoining in 1994.
At some point the list of countries whose citizens were eligible was extended to include Zimbabwe. I've been unable to establish exactly when this change was made; it may have been in 2002, when Zimbabwe was suspended from the Commonwealth, or 2003 when it voluntarily terminated its membership. It applied to rejoin in 2018, but by March 2021 it had not been readmitted.
From 2014, any novel "written originally in English and published in the UK or Ireland by an imprint formally established in the UK or Ireland" has been eligible.
© Haydn Thompson 2021