Highly persistent chemical warfare agents are not really what a military usually looks for. If something sticks to surfaces for a long time, you endanger your own soldiers, either when reclaiming lost territory or passing through newly conquered. Using long persistence chemical agents is a lot like salting the land, except also contaminating buildings, wells etc. one might want to use.
There is some debate about the persistence of Novichok agents, which could last several months. Also, Soman (GD) is pretty stable to temperature, but that's not the same as persistence in general. You could look into these more obscure agents, though, and make your mind up for your game. Literature is hard to come by, especially for the Soviet Novichoks.
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Liber et infractus
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