Would be very helpful for good statistics to have the amount of visitors
Like the idea too, even if it looks weird to track people saying "DNT" to us...
We have to make sure that this data is always anonymised and add a note about it in #6196 (PRIVACY.md)
Google tracks users with "Do Not Track" by default, however you can disable it, and you can also track, while disabled, how many visitors had DNT enabled. (see http://www.outfox.com/do-not-track-for-google-analytics/)
From reading the donnottrack.us website, it sounds like Piwik might theoretically be compliant with Do Not Track (even while tracking visitors) so long as it's not tracking cross-domain: http://donottrack.us/cookbook/
I wonder if Do Not Track support should actually be disabled by default in this case?