One of the main principles is that the routes have to be marked. Otherwise, another mapper cannot verify the route. We do not map planned, proposed or routes that are only available in books.

To distinguish between a local, national or international cycling routes, you have to know “how far” the route takes you. A circular route around a town is clearly local, a GR-path that extends from one side of the country to the other is national, etc.

Where I live, MTB routes get special markings and here and there you will find maps with those routes. An example can be seen on this photo on Mapillary
It a regional (Pajottenland is a region) mountain bike network.

p.s. there is a related discussion going on on the tagging mailing list: on recreational vs functional routes: https://lists.openstreetmap.org/pipermail/tagging/2020-January/050080.html