I must admit I have never met surface=smooth_paved yet.
I would rather expect surface=paved|asphalt and smoothness=excellent|very_good|good
To use your terms, surface=smooth_paved seems to me like an anarchic tagging :-)
Most routers, definitely all BRouter profiles I am aware of, would consider it as unpaved, as the tagvalue smooth_paved is not among listed paved values they use to determine pavedness.
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