OP here. To be clear, I did not violate their TOS. And deactivating an account for inactivity is not entirely unreasonable, especially a free account.
What is unreasonable IMHO, and the reason I want to bring this to people's attention, is deactivating an account without a warning, without even a notification, and with no way to recover. That is a ticking time bomb that I was simply unaware of. I never even imagined that a company as successful and popular as MC could have such an inane policy. Obviously, if I had know, I would have done things differently. All I can do now is sound the alarm so that other people don't step on this land mine. (And maybe public shaming will convince MC to give me my mailing list back, but I'm not counting on it.)
P.S. One of the problems here is that "inactivity" is ambiguous in this case. I had not logged in to my account in a long time, but people were signing up for my list. I assumed that would count as "activity".
The irony is that if MC had sent me an email saying that I needed to upgrade to a paid account in order to keep it active, I would have sent them a check without hesitation.
That's one of the frustrating things about this. MC has turned this into a lose-lose when it really didn't have to be.
That's the big takeaway here, in my opinion. Regardless of the fault with MailChimp (and I do think they're at fault here), you should never rely on a free service if it's mission critical to your business and there should never be a single point of failure like this. If this really was as critical as OP claims it is, then they should have taken more care to ensure that this wasn't even possible.
What is unreasonable IMHO, and the reason I want to bring this to people's attention, is deactivating an account without a warning, without even a notification, and with no way to recover. That is a ticking time bomb that I was simply unaware of. I never even imagined that a company as successful and popular as MC could have such an inane policy. Obviously, if I had know, I would have done things differently. All I can do now is sound the alarm so that other people don't step on this land mine. (And maybe public shaming will convince MC to give me my mailing list back, but I'm not counting on it.)
P.S. One of the problems here is that "inactivity" is ambiguous in this case. I had not logged in to my account in a long time, but people were signing up for my list. I assumed that would count as "activity".