For this question, I was extremely suspicious about the question, but I felt like I couldn't justifiably flag it as "Rude and abusive".
Is the claim likely to be complete nonsense? Yes, quite possibly, but potential nonsense is our bread and butter. In fact, I'm personally of the opinion that all potential nonsense ought to be asked only on Skeptics.SE rather than other sites.
Is the claim about a protected group of people? Yes, it's about Jews, but I don't think that should prohibit the question per se.
Is the question from an evil, hatemongering website? I'm not of the opinion that it's a disqualifying attribute, though others have differing opinions, but in this case it's not from such a website.
Is the question a duplicate? As far as I can tell, no. (I recently flooded the front page because of retagging spree I went on. Sorry about that.) The closest I could find was this one about Nazi Germany, rather than the Soviet Union.
Is the post not a genuine question? Unlike some recent questions that weren't really a question, but just a statement, this one was structured as a question.
Is the claim notable? Regrettably, I think it's a fairly common anti-semitic claim, so it is notable.
Is the claim trivially googleable, such that a genuine person could find their answer without asking? No, I don't think that's the case. It's kind of hard to prove a negative here, even if the USSR had antisemitic policies of its own.
Have lots of bad-faith antisemitic questions been asked before? Yes, but that doesn't mean this particular person was asking in bad faith. (Thankfully, most bad-faith antisemitic questions end up getting deleted - if you want to know more about that, see the questions listed in Require questions about the Holocaust to receive moderator approval before publishing)
Unfortunately, I don't have the ability to check whether the user was behaving in bad faith. I can't see if they've posted any previous rude and abusive questions on Skeptics, let alone on biology, meta, Mi Yodeya (the SE about Judaism), or philosophy, even though it seems odd for a user to have a series of 1-rep accounts with no activity on a series of different stack exchanges. (Update: The user has since been deleted on at least this web site)
I'm also unable to see whether this question is similar in structure to any other question that's been deleted as rude and abusive, because I can't search for deleted questions.
I don't want to become a diamond mod - it's a commitment I don't have time for at the moment.
I'm feeling limited in what I can do to investigate a suspicious question - all I could do was flag the question as merely potentially suspicious, and let a diamond mod check it out. (I was declined, though I know the moderators take antisemitism seriously)
What else can I do?