You would have to be a complete idiot to try and build your revenue on something owned by Twitter. They will shut you down at any time, for any reason, with no recourse.
> I'm saving the Substack mailing list regularly to my hard drive, and if I go somewhere else I'll let you all know.
So this is possible on Substack at least.
Some Substacks also use a custom domain name, which would make migration off the platform even easier.
> You own your list, forever > Bring the audience you’ve built, and take it with you anytime. You are in control.