For those who just need a newsletter with no paid options, Mailerlite might be good. (Please gods *don’t* use Mailchimp though.) Personally I prefer SendFox and got an AppSumo deal on it, but you do you.
I’m hearing about Buttondown and Ghost now too, though with the caveat that Ghost might be pricey. But better that than staying in a Nazi bar, you know?
Squarespace is pretty bad in some respects, but they’re at least a reasonably Nazi-free alternative and can function as a stopgap measure while one gets something else lined up.
Patreon shits the bed about once a year (though they usually back down with public outcry) and one can’t import subscribers, but they’re still an alternative. I like Gumroad despite some questionable decisions they’ve made.
Wordpress can work, since they have the paid subscription option that uses Stripe on the back end. SendFox is good for people who just want to put out a newsletter.