Team members can have their own opinion, of course. That is exactly what is happening in that thread, though Mazdak is the only language team member arguing that position. I personally wouldn't characterize this as "all development should be stopped until we have formal proofs," though, this argument is "if we commit to this API we will be locked out of one particular model and I'm not sure that we should be making that kind of decision yet."

Empirically it's pretty clear that the language is being developed. If that were the decision of the team, it would pretty much have to go through an RFC, and so we'd be able to point to that.