How should we tag this? It seems like a useful thing to have on OSM. I can’t seem to find an ID from the Highway Code that I’d be able to put in a traffic sign tag.
I don’t think you need to, however the reason for asking cyclists to slow is worth mapping for example if it’s a chicane barrier or a road crossing.
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Just ignore them the same way actual cyclists do /ducks/ 
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Some of them are due to corners with minimal visibility - but yep, it’s usually crossings, which are already there.
If you wanted to micromap the presence of such markings you could add a node with road_marking=text + inscription=SLOW
The node would be placed at the location of the marking rather than connected to a highway way, so it would still be hard for a router or consumer to actually interpret that data.
Documented in Key:road_marking - OpenStreetMap Wiki
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If you can identify the reason for the SLOW marking, you could also tag that segment of the cycleway with hazard=*.
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