Have had a 20+yr old coop with unpainted OSB floor and now a coop with painted floor. Both setups were fine with deep bedding (8"+ deep) as the floor never really got wet by anything. Even when I hosed down and scrubbed the bare wood coop it was never a problem. It's more constant moisture that kills wood.
If you have a leaky roof, waterers inside, high humidity etc. that will maintain a certain level of moisture on the wood surface so it never dries out, then your wood is obviously more susceptible to rotting and a solid surface is a good idea - linoleum, blackjack, redguard, enamel paint, perhaps even peel stick tiles, etc.