You can get an idea of which sites are blocked by reviewing the list of blocked domains [1] (this includes many APIs, such as reddit.com) and a 5 year old issue where people post websites broken by tracking protection [2].
[1] https://github.com/disconnectme/disconnect-tracking-protecti...
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1101005
You can get an idea of which sites are blocked by reviewing the list of blocked domains [1] (this includes many APIs, such as reddit.com) and a 5 year old issue where people post websites broken by tracking protection [2].
[1] https://github.com/disconnectme/disconnect-tracking-protecti...
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1101005