Which of your products do you view as most likely to have led to the termination?
I don’t mean, which do you agree or consider just cause for leading to it. But as the operator of them, you are best positioned to estimate which one(s) a risk-avoidant corporation would be most likely reacting to, and knowing more about your actual circumstances would be valuable anecdata for others trying to help (as well as those reconsidering Vercel!).
This is one reason why I avoid Next.js. Don't design your future projects around specific platforms. Things happen. Make sure you can pick up your code and go somewhere else.
Edit: Strange I would get so many downvotes for this. Care to explain? I think the effective Next.js lock-in is well documented. Yeah you can technically run it on arbitrary platforms with substantial work and continued maintenance, but you're working against the tide.
Maybe Vercel is pivoting away from your use case to focus on larger organizations.
Maybe Vercel lost the data.
Maybe Vercel saw a regulatory issue.
My advice is to keep building on a different platform.
Good luck.
What a disappointment to hear
More and more companies have zero meaningful support when things go sideways
I'm guessing one or more of your sites allowed user-uploads in some fashion or another and someone put a file that is hash-flagged in a serious manner
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There are some types of legal action that service providers can get, which both force the service provider from droppig you, and also forbid the service provider from telling you why or what has happened. Eg, this happens in banking with SARs; when they drop a client for suspected money laundering, not only can't they tell that to the client, they can't even tell their own support agents.
I don't know anything about your sites or your activities, but based on the behavior described, something like this has probably happened to you.