Discord and Reddit in this case, even if they're not banned from Reddit yet, is their only means of communicating with eachother. Sure people can create some kind of group / channel on another platform, but if they are banned from one there is no way for them to know where to go.
Obviously the group is going to be mostly dispanded when they are banned or removed. I would say this is a kind of censorship. If they would've been given some time, like a week or a month before the banning then I would agree it would not be censorship because then they can assemble somewhere else.
All journalists, nearly all politicians, and lots of other people as well communicate largely on social networks owned by these giant amoral artificial creations. What does "public space" even mean anymore? Should we stand on the stroad median with the panhandlers? Would anyone even read our signs?
It is possible for citizens to impose conditions on the existence of the corporations their elected government creates.