The supreme court has ruled time and time again that the right to free speech implies the right to be heard.
While CloudFlare obviously isn't legally required to serve 8chan, the only service they are really providing is making it so that 8chan can't be DDOS'd. And DDOS'ing is very clearly a tactic that violates the first amendment. So really by no longer serving 8chan, the only thing they are doing is allowing and/or encouraging others to violate 8chan's constitutional rights.
It reminds me a lot of Charlottesville, where people showed up to try to shout over the white nationalists and it just ended up with a bunch of people getting run over by a car. As if something like that happening wasn't entirely predictable.
If 8chan is inciting violence or doing other things not covered by the first amendment then it should be the government's job to police that.