3rd party cookies work less and less and eventually won't be available anymore. Kind of related post https://matomo.org/blog/2020/02/new-cookie-behaviour-in-browsers-may-cause-regressions/ For the opt in to work we therefore need a different way and set first party cookies.

Matomo for WordPress already doesn't use the opt out iframe anymore and sets first party cookies. In On-Premise as part of #12767 we already added the support of postMessages to set first party cookies when possible. This however currently only works in some cases (eg when the tracking code is embedded on the same page and both opt out and tracking code use the same Matomo domain).

In the future ideally we show a message in the opt out iframe when it won't work because eg there's no tracking code on the privacy policy page. We might even want to completely remove the third party cookie part (however we'd still need to detect it when it's set and not track to not break BC and to not suddenly start tracking users that oped out previously). Maybe we could even remove the domain check and opt users out in more cases even if there is a mismatch between the opt out iframe and the tracking domain on the privacy policy page.

Or maybe we would need to offer a new way of embedding the opt out without any iframe. This would likely require loading another JS and some configuration to customise it and for Matomo to know where to place it (unless this is all stored in a JS file and the user can configure multiple different JS opt out files).