Substack’s docs show an option to export email addresses—I would presume that would mean you could move your subscriber list to another email platform if you wanted to?
Followup here: I double-checked with some others who publish with Substack, and [you can export your Substack subscribers](https://capiche.com/q/what-did-you-use-to-start-a-paid-newsl...). At least that keeps the lockdown a bit lower (though of course you would have to get subscribers to pay again at a new site, which would likely lead to high levels of churn).