Over the past couple of days, an https://thenewstack.io/rich-harris-talks-sveltekit-and-whats-next-for-svelte/ about the next major version of Svelte blew up on twitter, and caused lots of discussion. Technically, the article is not wrong, the team is switching the underlying code from TypeScript to JavaScript.
They're just moving from writing TypeScript source code directly, to writing JavaScript source code using JSDoc in combination with.d.ts files.
What's interesting about this discussion is that a lot of people found this to be very upsetting, and twitter blew up with discussion about typechecking.
These projects still utilize TypeScript to ensure typesafety in their code, and can still ship types to their end users.