My favorite knowledge sources as a developer
Dev.to is a wonderful place for developers. You can subscribe to language and technology topics, and scroll down your custom feed that includes tutorials, debates, videos, personal projects and lot of interesting resources. I love it because most of people are humble and have the sense of sharing. Of course you can also fill your reading list if you are in a hurry, and the navigation is rather pleasant. It's time-consuming. If I had to pick a single one, it would be Dev.to.
GitHub is not really a newletter, but the Explore tab can be interesting because it features repositories based on your interests. Reading other's code is a nice way to introspect your own work and discover new things. There could also be new libraries for your projects.
Medium Daily Digest
I used to read Medium more often, even if it's now requiring a subscription (you can still open links in a private navigation tab). It may be definitely less "technic-centric" and more general, but I see a lot of "how to be a good person"-like self-promotion posts that I don't really enjoy reading. There are some nice publications though, but it's too bad that Medium is no longer free.
Vue.js Developers, React Status and GraphQL Weekly
React Status, Vue.js Developers and GraphQL Weekly are good weekly newsletters about their respective frameworks. I'm more into Vue.js than React.js, but I read some pretty interesting things about the latter, that made me want to play it. GraphQL Weekly is more about videos and introducing libraries, there is lots of knowledge to learn from this young ecosystem.
Of course there are many newsletters and websites on the internet, these ones are just my favorites.