I don't need a Twitter account to read people's tweets, and I contact my representatives through phone/mail. If they decide to spend time in a private platform not everyone will be able to message them on (or spend a lot of time in a restricted physical location that only lets people with "correct" opinions in), that's fine. If a politician wants a consistent way for everyone to be able to read their writing and contact them, there's nothing stopping them from just... Using their own website for that.

Just because some "famous" person uses a platform shouldn't mean the platform has to fall over itself to make sure their every word is hear and everyone can contact them.