I would bet S0 will not get WatchOS 5 no matter what. S0 processor is way too slow.
If it does not receive watchOS 5, that would put the first generation Apple Watch at approximately 3.5 years old from its launch in April 2015 if a Series 4 launches in September. Even if it doesn't receive watchOS 5, it still has its functionality, but the processor certainly affects its performance in some areas.