As far as I know, there was an attempt to apply OSM’s coastline processing pipeline to OHM, ignoring differences over time at the outset. But the effort stalled because there were a lot of broken coastlines. Maybe some of the breakage is due to variations over time, which the software doesn’t account for, but we’d need someone to take a look and understand the prevalence of actual mapping errors.
I don’t know what the plan was for rendering the variations over time. Maybe our more recent work on boundaries would be more straightforward to apply to the coastline problem, obviating the earlier work. After all, OSM adopted the natural=coastline tagging scheme and an extremely fragile coastline processing step so that mappers wouldn’t have to maintain equally fragile relations. But we unavoidably maintain boundary relations spanning continents and beyond. Alternatively, maybe the hack we’ve been using isn’t such a hack after all, if we can convert the space in between the coastlines into bite-sized landmass areas or the inverse at sea.