It needs to be for good reason, it needs to be in a transparency report by the relay operator, otherwise it's a "shadow ban". The reason could be any number of things, CSAM? DMCA? Law enforcement requirements? "I don't like what you're saying" is clearly not a valid reason.
Does it matter if it's "shadow banning" when practically speaking you can't get banned from the network as a whole anyway?
Imagine you've been banned from Gmail, Outlook, iCloud and Yahoo all at the same time. Your messages would still be available across the Email Network, but most (yes, that's right) email users would no longer be able to receive messages from you.