You have a right to speak, but not a right to reach.
The majority of speech always took place in areas with some kind of limits. For instance: in some guy's tavern or in the pages of a local newspaper.
Furthermore, what's happening to Parler could also be conceived as a kind of self-defense exception: the factions it embraced have recently attempted to literally attack (in the name of a selfish demagogue) the heart of the liberal order that enables free speech, and they cannot be tolerated if toleration is to survive.
The majority of conversation in democracies has always taken place in private venues that were free to control who had access and did absolutely do so based on political viewpoint.
That these private spaces are now virtual rather than physical doesn't change the essence of that fact.