First, the post from the conference staff on the matter. They went to quite the lengths to come to their decision, as you can see if you read through the whole thing.

Then, some background on what the hell is Moldbug guy about anyway. In short he's a neoreactionary (a genuine one, not the reddit insult version that's code for "someone who I perceive to be right of me politically") famous for writing massive walls of tl;dr on his blog Unqualified Reservations filled with various kinds of controversial ideas about how democracy is bad and whatnot, which has naturally resulted in the always polite social justice crowd calling him a neo-nazi, racist, fascist, and many other charming words.

Last year this crowd actually did get his talk axed from another programming conference, StrangeLoop, on the basis that he's a bad unperson because of his politics (even though they have nothing to do with what he would have talked about), which is why you can probably imagine their disappointment when LambdaConf decided to go the opposite way, and this is where the drama naturally kicks into high gear. A sampling of spicy tweets for your consideration:

Wow LambdaConf is now on my list of conferences to NEVER attend

Fuck this bullshit

So Moldbug will be speaking at @lambda_conf, in case you're wondering why I won't be speaking there after all // Maybe @jdegoes can afford to be philosophical about this, but as far as I'm concerned this is a political statement. And fuck that shit. To put it politely.

"we asked a bunch of ppl who stayed thru hostility and acceptance+defense of overt discrimination and they're cool with the neo-Nazi" OH OK

It’s now perfectly clear, lot of functional programmers are just fine with having an outspoken racist at a conf. Good job, keepin’ FP great!

ur b it is a bad lang because the creator is a fascist. the design sucks bc the designer can only think in terms of conquest and violence // every lang uses analogies, but the analogies there are polluted by violent authoritarian thinking in ways that make it technically inferior [TIL a programming language can be fascist]

Observe numerous maggot racists eating the corpse of FP pupating into their filthy adult glory https://news.ycombinator.com/item?id=11346124, thanks @lambda_conf! [The HN Link for one-click reference]

(So what does he want to talk about on programming conferences anyway? Urbit, which is basically his general philosophy of "the current system is fucked and we must rebuild it from scratch" applied to systems & programming, delivered in a very deliberately obscurant manner.)