In fact they are called "Public GPS Traces" and can be enabled in the OSM layer inspector. I am not sure if anyone started distilling track recommendations from them yet though.
That's possible. I use RideWithGPS, maybe a lot of people use it around here. Or maybe only two other people have ridden the route, it doesn't say, but it's still super useful for finding routes.
Maybe the data could be shared/distributed via hypercore or similar.
I find heatmaps are my primary way of finding new mountain biking trails and routes.