I'd argue there is a difference between a blog and a newsletter, but that would take longer to discuss than I have right now. But you're right that the boundary between the two is extremely blurry.
However, I'm guessing you tried to submit a Substack URL and was told newsletters aren't accepted? Even if I allowed newsletters, I wouldn't add any Substack newsletters because of the platform's fascist-supporting tendencies.
However, I'm guessing you tried to submit a Substack URL and was told newsletters aren't accepted? Even if I allowed newsletters, I wouldn't add any Substack newsletters because of the platform's fascist-supporting tendencies.