I think you (and maybe also the golem author a bit) are confusing third-party-scripts (JS that is loaded from a domain, that isn’t the one you are currently on) and third-party-cookies (domains that are “stored” on another domain than you are currently on).
If you embed Matomo tracking from
example.com, you are loading a third-party-script, but the script is executed in your website and is therefore able to set a (non-third-party) cookie to
example.com (which is what Matomo has always done).
Therefore Matomo is not affected by blocking third-party-cookies. (BTW, the same is true for Google Analytics)
One part of Matomo, that is affected by this, is the opt-out iframe. if you embed the opt-out iFrame from
matomo.thirdparty.example and is therefore only able to set an opt-out cookie on
matomo.thirdparty.example. If your browser is blocking all third-party-cookies, then the opt-out can’t be set because the iFrame can’t store anything.
One solution (technically the only solution) is not running the opt-out in an iFrame and instead in your website.
For that you can use the (fairly new) custom opt-out feature of Matomo:
If you execute
_paq.push(['optUserOut']); the tracking code running on your domain is able to set an opt-out cookie on your domain and again is not affected anymore.
The side effect is that the user is only opt out of
example.com and not all websites tracked by
I hope everything is a bit more clear now.