I say map everything! Something like "flood risk" would be a valuable part of a navigation network. No telling whether or not these features are in Google / Apple maps's networks, as those are closed-source, but it's common practice to add anything of this nature to OpenStreetMap using the hazard tag.
For the road in your example, I'd map the sign itself, to get its accurate position, but also add the hazard=flooding tag to the section of the highway feature as well. This still doesn't guarantee that the tag is actually used in a given routing tool, but the fact is it could be, and adding the feature enables hazard-aware routing.
Other tags, like hazard=animal_crossing or hazard=landslide can prompt a routing service to alert the user "In X miles, watch for falling rocks", "for the next X miles, be alert for animal crossings", that sort of thing.
- Josh Carlson
Kendall County GIS