And most of our tags on Commons have no meaning for the external world, for instance last time I checked we didn't use the public domain mark for public domain files.

We actually do, on about 2.6M files :) [1]

(Transclusion of this on our PD templates is probably not consistent though, and maybe not justified in some cases.)

[1] <https://commons.wikimedia.org/wiki/Category:CC-PD-Mark>

Jean-Frédéric