Source code: https://github.com/zgoat/goatcounter.

The author also gives a rationale for choosing the EUPL (EUROPEAN UNION PUBLIC LICENCE) here: https://www.arp242.net/license.html.

EDIT:

ksec posted this link: https://github.com/zgoat/goatcounter, but, at the time of writing, my comment is higher...

It provides rationale for why GoatCounter exists and comments about _why not_ other solutions like Fathom, Open Web Analytics, KISSS, Ackee, Countly, Analysing log files, Google Analytics, statcounter, Simple Analytics, getinsights.io, statcounter.com. plausible.io/.