Is there a right to have your voice heard on a private platform? If you send them something they have a right to publish it but do they have any obligation to do so? I don't think so.
The left wants to promote the notion that censorship is not censorship if it can be done legally or if it isn't government doing it.
You propose this question of if there is a right to have your voice heard on a private platform, but rights violations are not relevant to the meaning of censorship. Blocking and removing voices from private platforms is censorship, regardless of any rights.
Declining to publish something may or may not be censorship, each situation is unique.