Like I said, I can totally empathize. No, it's not cool to do that without any notice or consent. You didn't get the notice so as far as you're concerned there was none. I get that, it's totally understandable.
That said, I don't think your example of Twitter is a good one. They're too different for that to work. But I don't think getting into that would be very productive for anyone so rather than doing that I'll just say this: while I get where you're coming from I think you're expectations are just north of being totally reasonable. The core if it makes total sense but it starts to unravel when you consider your expectation that you want to " trust them with the responsibility of not altering our relationship in a material way relative to the terms of service of the moment when I signed up for their service". That right there, the way I read it, means either "I expect your company to provide the exact same service and follow the exact same policies that I signed up with forever" or "you can only change your service and/or policies when it benefits me". That's unreasonable. No company on earth keeps its policies and services the same forever. They change every now and then. It upsets people and sometimes a change is negative for us but we adjust and move on.
I get why you're leaving but still don't think Disqus is in the wrong. They obviously need to change their service to continue to provide the service. For some, like you, that's not acceptable but its still not that big of a deal. One thing I think you're forgetting is that there's a huge gap here between what you experienced and what the reality is.
What you experienced what a major change in service without your consent or knowledge of it at all. That's certainly a reason to be upset.
But the reality of the situation is that your case is an exception. Disqus did provide warning and a way to opt out. Now, had you not missed the announcement about this would you still feel the same way today?
To be clear, I'm not telling you you're wrong to feel this way. What I'm saying is that it's wrong to characterize Disqus' actions as being sneaky or in breach of trust because they did provide a warning and a way to opt out - you, unfortunately, just happened to not get the memo. That sucks.