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.
In principle, sure. But in practice for many politicians, sending a letter will receive a copy-pasted reply while replying on twitter might get you a personal response. Similarly, getting kicked off Instagram will cause many businesses to die, despite there being another dozen sites where you can upload pictures. Social networks have an inherent tendency to monopolise niches, even if those niches are hard to articulate.
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.