You can subscribe to newsletters with any RSS reader nowadays, but it’s a multi step process requiring the use of an external (free) tool.

1. Create a feed on https://kill-the-newsletter.com/. This will also give you a custom email address to send your newsletter to.

2. Subscribe to the newsletter with the custom email address and add the feed you created to your reader.

This setup works very well for me with NetNewsWire though there is friction in the multiple steps. No affiliation with either, just a satisfied user.