Would you rather have an RSS feed than another newsletter but the site doesn’t offer one?
Kill the Newsletter is a great service that gives you a unique e-mail address (for subscribing) and generates a unique Atom feed for your reader, containing all delivered newsletters.
Stay private and updated.
https://kill-the-newsletter.com
Just realised I wrote a quick guide to Kill the Newsletter in 2019, with an infographic. 😊
“I found a useful tool allowing me to partake in the joy of well-written newsletters without having them fill up my inbox. The website Kill the Newsletter! (KTN) simply allows you to add them to your feed reader.”
https://axbom.com/newsletter-rss-atom-feed/
And of course, I just discovered Substack (specifically) doesn't allow email addresses from Kill the Newsletter. Another reason to dunk on Substack.
[update: as Dale Hagglund rightly pointed out on the comments, Substack has RSS feeds if you add /feed to the newsletter url. Note, though, that if it's a paid newsletter and you subscribe only to free updates, the feed will contain links to paywalled updates as well, that you may wish to keep out of your feed]
You may have to create an alias for your email address. If you're not familiar with creating an alias you may want to try one of these services:
https://simplelogin.io/
https://burnermail.io/
https://relay.firefox.com/
https://anonaddy.com/
https://altmails.com/
https://duckduckgo.com/email/