What is the map scale for the shapefiles downloaded from GEOFABRIK?

Shape files do not have a map scale. A map scale determines the relationship between units in reality and units on the map. If you have a map that is 50cm wide and it depicts 50km in reality then its scale is 1:100k; a shape file does not have a scale because you can print it at any resolution.

By convention, certain scales also go along with a certain selection of features. For example, a 1:100k map will usually not show individual trees (even though it totally could, if it made the top of the tree about 0.1mm wide). When you ask "what scale are the shape files in", you probably want to know if there is a certain scale-dependent feature selection or generalisation in the shape files.

The answer to that is no, the Geofabrik shapes have no generalisation or simplification, and the feature selection is based on a very simple tag selection. For example, anything that has an amenity, tourism, shop, office, craft, historic, man_made, railway, or highway tag and is a node will be listed in the "points" shape file. The means that individual trees aren't available (they are natural=tree), while post boxes are (amenity=post_box).