Years ago, before email as we understand it today existed, I roasted and sold coffee and the newsletter drove close to 80% of the sales.
Years ago I built a small "adult entertainment" business that was driven by a newsletter. High open rate, high engagement, strong sales accounting for a significant percentage of sales through the month.
Years ago I built a piece of software that made stock buy/sell recommendations. It was driven purely by a newsletter. The daily recommendations email went out once per day, very early in the AM. Once a week was an email with a link both at the top and bottom of the newsletter that you had to click to stay subscribed, and that newsletter had a little lengthier content in it. The newsletter was by invite only. It had good engagement. I got tired of running the business (wasn't very interesting) and I sold it off, and the new owner drove it into the ground within six months due to greed, making the newsletter less exclusive, and changing how engagement works and trying to use it as a vehicle to push advertising and other bad practices.
Years ago I built a piece of software that analyzed horse racing, and again, it was all driven by newsletter, and that accounted for all the engagement, most of the sales, upsell/cross-sell and subscription renewals. I sold the business after a couple of years, and watched the new owner drive it into the ground due to greed and not understanding what the newsletter was for.
I've also built a few other small side-businesses over the years, using long form content, long form sales pages and newsletters as the primary sales channels. They work wonderfully. But what you absolutely have to do is realize the value is in the mailing list, and it must be respected, and you cannot AB test it or exploit it to extract maximum value.
I still use newsletters, of a sort, for a few side-projects, and again, they work really well. Very high engagement. Especially on LinkedIn.
Of all the newsletters I've had over the years for various endeavours I've had a very high open rate, a very high engagement rate, and an incredibly low unsubscribe rate. But you have to treat the person receiving your newsletter with the same respect you would treat a well regarded acquaintance standing right in front of you. And the moment you stop doing that, your numbers will plummet.