Ayenson et al. reported in their paper [Flash Cookies and Privacy II: Now with HTML5 and ETag Respawning] years ago that Kissmetrics used ETags to identify clients.
When looking at a site which uses Kissmetrics, for example http://www.up.edu.br/, I can see the
ETag element in the response header when loading the Kissmetrics' tracking library.
However, I do see the exact same identifier when browsing in private mode.
So, are they using ETags again?