UPDATE FROM STRAVA:
Thanks so much for writing in and providing this feedback. This is something that we are aware of and currently looking into. We're actively working on solving this gap in user experience and weighing this feedback into consideration as we work on an update. We appreciate your time and patience.
a recent feature seems to have filtered heatmaps based on the sport type when routing. i suspect that this is broadly a great feature, but it has now made gravel routing very difficult because of the data sparsity (a large % of people don't select "gravel ride" on their activities, most rides aren't gravel anyways, etc.). plus there's so much overlap in gravel and road routes anyways, so aggregating that data makes sense imo.
the road heatmaps work great here, but the only way to get them to show up is to change the sport type to road cycling, which affects the routing. I prefer to use "gravel ride" as the sport to get good autorouting ("prefer dirt" has its issues since it always wants to send me on hiking trails instead of just doing the logical thing..). most of the heatmaps around me are pretty much empty for gravel routing, so my workaround was to switch between sports when routing so I can view the heat map, but this became a pretty crappy workstream to make routes. im having to move to ridewithgps until this gets cleaned up. im pretty bummed too, because ive generally preferred Strava for routing, and I now have to learn a new workstream for the time being.
my suggestion would be to allow users to toggle data for the heatmaps (eg I can show an aggregated heat map of road, gravel, mtb activities) when routing.