I’ve been looking for one of these for some time. Like many autistics, I get distracted easily, and often put off writing while I fiddle endlessly with the formatting, etc. Writer is a stripped-down online word processor that (pretty much) only allows you to type. There’s three fonts and line-spacings to choose from, and you can pick the colours for text and background. That’s about it. You can also choose to make it sound like a manual or electric typewriter, which helps to set the writerly ambience. Text can be exported for formatting elsewhere. Also available as a Google extension.
Writer: The Internet Typewriter