Script is default and at the page botoom, after website UI scripts, but until its load failed all documentReady functions waits for it.
Not sure if I got your report right. The async script loading of piwik.js blocks your documentReady event on safari? Is that reproducible for you on any other browser?
I tried Chrome and Firefox, last and updated, they are ok, it was only Safari. The matomo server was dead (something was happend on digitalocean), and i saw this problem. I will try to make this issue more informat in free time, because to reproduce it need the server that will not response and will keep connection.
In case someone can reproduce this issue in the latest Matomo version, please create a new issue with more details about what doesn't work as expected.