About Tithe Taker, this appears to be the source.
This is a person with not many followers, relative to other Magic accounts that you'd expect to have preview cards. They didn't present it as a preview card, but rather had a series of tweets with the same format for the other cards revealed today, and this was just among them.
If people were getting preview cards this week, where are the Rakdos cards?
Like I mentioned before, this uses art we already knew about. The mechanic seems much harder to develop for than what Wizards typically goes for.
We can see in Evan's most recent tweet that the reminder text's use of "a" instead of "one" means the afterlife number should refer to the power/toughness, which is weird as fuck and definitely not what Wizards would do when +1/+1 counters are right there.
All of these are big red flags. Of course, we'll know for sure this week, so no big deal.