And we keep having these discussions where we all agree "It's not censorship when it's a private company" but that's starting to feel like a truism we need to push back on, because if every private company that handles mass emailing decides to stop accepting a certain type of content (let's all agree we aren't talking about child pornography as the content), that is censorship. If this story were, "T-Mobile stops allowing text messages/ phone calls about blockchain" I think the reaction would be different even though they are a private company as well.
Email is an open infrastructure, if an consensus emerges among the existing players not to allow emails about X then you're free to start your own email service what will feel the gap. If you can't make it stick then the holy free market has spoken.
Wikipedia is currently blocked in Turkey. That's censorship. Mailchimp won't let you use them to send emails about Bitcoin. That's a minor inconvenience.
If every email service prohibits a topic, that's indistinguishable from state censorship. I fail to see how only one is bad and the other isn't.
The free market isn't of any help here either. Censorship is about stopping the spread of information and ideas. What's not known can't affect the free market.
Email is nominally an open infrastructure. In reality, antispam measures and companies can render you unable to send messages in general if you annoy them enough.
Spamhaus in particular seems to have a reputation for unreasonable practices and rather unpleasant-to-deal-with people.
T-Mobile isn't a good analogy because it functions as a utility and has very few competitors. MailChimp has many competitors, including "spin up your own mail server" and "use a different marketing channel".
