Would Voltaire have staked out this absolutist a position on free speech if he'd lived to see things like the Holocaust?

Why would the holocaust change his opinion on free speech?

1. The fact of the Holocaust happening under the leadership of Hitler (a deranged man who captivated audiences with charismatic and anti-Semitic speeches, played on Germans’ fears of victimhood, etc. etc.) shows that when hatred is given a platform, a certain percentage of people will listen and be radicalized;

2. The fact that there continue to be Holocaust deniers and downplayers to this day shows the enduring and damaging power of rumor and innuendo, despite ample historical evidence to the contrary.

I don’t think Enlightenment-age political philosophers like Voltaire really reckoned with these problems or had any reason to reckon with them.

1. I support the right to freedom of speech, but not the right to demand a mass genocide.

2. Holocaust deniers are just expressing their right to freedom of speech.

And why do you get to speak for genius philosophers like Voltaire?

Basically

A freethinker like Voltaire would expect no less from us

In my opinion