From the Find page, if you scroll down, on the bottom left you will see a box checked for All Recreation Types. Uncheck this, and a whole bunch of other types of activities will display, including Mountain Biking. Select that + any criteria you would like and then search. This will only display routes/rides that have been saved by members as Mountain Biking, so likely not everything. Your best bet might be to use the Explore Maps feature, as demonstrated in the this video:
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