These are just methods of injecting fringe ideas into mainstream discourse to make it seem like 'normal' people think these things. A lot of this just functions via repetition, see also the 'Big Lie' technique, which has a history going back quite a bit further than last November.
I tend to view everthing with a degree of skepticism, and I tend to reject the idea that there are subjects that are "off limits" for critical evaluation.
I'm not sure how that's enabling fascism. It seems like it would do the opposite, no?
The problem is that that can't actually be true. If it were, you wouldn't be able to actually do anything. You'd be too busy being skeptical about whether the sun will actually rise tomorrow or whether all of the people around you are actually simulacra. It's an old and well-discussed problem called Radical Skepticism:
https://en.wikipedia.org/wiki/Philosophical_skepticism
You can expand "degree" to re-enable your ability to act, but that's exactly what's going on here. They're saying, "No, we're not going to waste more time re-re-re-reevaluating often-refuted observations".
I'll set aside tendentious arguments about what constitutes "fascism", but it's definitely the case that people use insincere arguments for "skepticism" in order to shut down all genuine inquiry. That's not productive, and it's useful to find ways to circumvent that.
But it turns out that most of us want to have strong beliefs after all. So, after our commitment to democratic principles (along with everything else) has been weakened due to the skeptical paradigm, we are ripe to have other strong beliefs take the place of those which have been dethroned. Enter fascism, evangelism, and other belief systems with highly dedicated cadres. They're like opportunistic infections which take root when our natural bullshit-detecting immune systems have been destroyed by a diet of excess skepticism.
I strongly disagree. There is an enormous shift from "I question whether Jews were killed" to "Let's go kill Jews".
(To avoid misunderstandings and red herrings: Yes, the Holocaust happened, and was very much as horrific as claimed. The evidence is clear, consistent, overwhelming, and almost impossible to deny. And there are almost certainly recruiters who use "just asking questions" to try to recruit people. Nevertheless, "shut up and stop asking" isn't the answer. Instead, the answer is to show evidence, in overwhelming amounts. This may not silence the recruiter, but the recruiter isn't the target of your evidence. The recruitee is.)
You said "tolerating fascists" rather than "being a fascist". But it's still an enormous shift from "I question whether Jews were killed" to "It's OK that you want to kill Jews".