I primarily use feeds to stay on top of interesting authors and websites. I seldom need to deal with newsletters because almost everything I follow (including a few Substack and Buttondown newsletters and the newsletter for Tablet Magazine) also have RSS feeds. But there are a few exceptions, notably some of the Real Clear network of sites. I started using Omnivore to cover this use case since it supports subscribing to newsletters from inside its app with its own email addresses. Blogger Dan Q here wrote about using an Open Trash Mail instance to handle newsletter to RSS conversions. Apparently one can subscribe to these disposable addresses via RSS. I’ll keep this in mind for future reference.
No, I won't subscribe to your newsletter... except by using my RSS reader (and a clever feature of OpenTrashMail)!