Since I assume there is no recourse for these users on a censorship level, is there such a thing as a class action lawsuit for removing a feature in Hong Kong?
Nonsense. The existence of proprietary platforms in no way reduces the possibility of free and open platforms. Using iMessage or Facebook Messenger or any other closed communication platform is a choice you can opt out of right now.
Sometimes I wonder if this all-too-common "the open Internet is dead" defeatism isn't some kind of false flag to make people think the open Internet is actually dead so they don't try to leave their walled garden. That is absolutely not the case and people need to stop saying it.