This isn't a blow to free speech. It's Twitter deciding not to let someone use its service and its resources for the purposes of rallying racists to berate an innocent woman who acted in a movie.
On the contrary, private censorship is essential to the marketplace of ideas, since it is exactly private parties choosing how they will participate in that marketplace.
(Where a "private" entity -- or a group of them coordinating formally or informally -- has a, de facto or de jure, protected monopoly on communication in a market, then that entity or entities censoring raises much the same issues as government censorship, but private censorship in general is central to, rather than destructive of, the open marketplace of ideas.)
Why should I protect someone calling Leslie Jones an ape? And do NOT say to simply "ignore it" or to "log off". None of that works. It doesn't stop it, and asking someone in the public eye to not use social media is basically telling them to not have a career.
Banning someone from Twitter is one of the least effective ways to censor an author who has multiple venues which pay them to write. As the linked comic says (I can't believe nobody has linked to it yet, so the obligatory XKCD is below) this is not censorship. This is signaling that his speech was inappropriate and harassment. Being banned from Twitter doesn't stop him from posting on e.g. Facebook. His speech can still be heard by any who want to hear it, this ban merely prevents him from harassing her further.