I think a lot of people are completely oblivious to it, simply because Jews are a tiny minority in this country and as Jew-hate takes a different form to more conventional racism that we learn about, it needs reading up on to understand.
Over 20 years ago I read a book by Jon Ronson (who is Jewish) called Them: Adventures with Extremists where he spends some time with various groups with extreme views including an London-based Islamist who raises money for Hamas and Hezbollah (pre 9/11 and he has since been kicked out of the country), a journalist hunting down the Bilderberg Group, David Icke and Aryan Nations. And I was shocked that these very different people with different extreme views all had issues with Jews (except Icke insists lizards, but those around him probably don't). It wasn't something I had even given any thought to.
When Corbyn became Labour leader, a political commentator I rate who is Jewish started talking about the antisemitism in Labour. So I looked into it, spent some time on twitter and what I found was absolutely disgusting. On the surface you had 'coded' stuff like the Young Labour account tweeting 'Free Palestine' at Jewish MPs, or the 'Israel Lobby' controlling Tom Watson and the despicable treatment of Luciana Berger, but you just needed to scratch the surface slightly to find blatant holocaust denial and 'Jews control the world' conspiracies. I felt physically sick reading it so god knows how Jewish people felt. And then I'd come on MN to find people asserting that there was no antisemitism in Labour and it was all being fabricated (by, guess who?) to discredit Corbyn.
But if I hadn't looked into it myself, and been aware of what to look for due to prior reading, I think it would have just been some background news thing that I'd have been aware of with 'one side asserts x while the other side strenuously denies'.
I find it quite worrying that people are wandering into support for the people of Gaza (which is obviously much needed) while being completely oblivious to the antisemitic discourse which surrounds Israel. If you don't know that 'Zionist' has been used as a code word for Jew by antisemites for years, you wouldn't necessarily be alarmed by the demonisation of Zionists on placards and in general discourse about Israel. You can see on MN that a lot of people assumed that Zionist meant 'supporter of Netanyahu' because of the way the word has been used to mean someone despicably evil. Because they didn't know to be wary, and they don't realise what is really going on.
Similarly they wander into the assumption that people have an issue with Israel purely because of the things that Israel has done (which I don't support) and not because Israel is a Jewish state. See chanting support for the Houthis because they 'support Gaza'. They may be surprised that the Houthi slogan includes 'Death to Israel and a curse upon Jews' but they're probably never going to look into it deeply enough to find out. I've also seen on MN the assertion that Hamas are not antisemitic, the holocaust was a European thing and that the Middle East never had any issues with Jews until Israel was created. Which, if you look at the history, is just obviously wrong.
And if you support Gaza, it is very easy to be exposed to these views without realising what is at the root of them. It's a form of grooming, in a way.