I searched a bit through the documentation and code, and these were my findings. I thought I'd share them for others that are interested and for future reference.
Currently, there are no domains blocked, they would appear on this API endpoint: https://firefox.settings.services.mozilla.com/v1/buckets/mai...
This is the JSON schema for this API endpoint: https://firefox.settings.services.mozilla.com/v1/buckets/mai...
More information on the remote settings in general: AMRemoteSettings Overview - quarantinedDomains: https://firefox-source-docs.mozilla.org/toolkit/mozapps/exte... Remote Settings documentation: https://remote-settings.readthedocs.io/en/latest/index.html
Remote Settings DevTools - where you can see all the remote settings, that get set: https://github.com/mozilla-extensions/remote-settings-devtoo...
EDIT: Seems like there are many settings that already get automatically set via AMRemoteSettings (including search-engine configs, cert revocations, dns over https providers, password rules for specific domains, top-sites, URL tracking parameters to clean, etc.). We will see how this new setting will be used, it can be easily disabled (https://support.mozilla.org/en-US/kb/quarantined-domains) and you will get a warning if an Add-On is blocked from accessing the site. Also seems like there will be a UI for this in v116 (https://bugzilla.mozilla.org/show_bug.cgi?id=1837670), where you can configure this better than just disabling this feature completely.