Trying to turn this useless feature off just took me far too long - and I'm sorry, but it is useless. Aside from the markup / special characters toolbox being gone, restricting the input box to rich text with like 4 formatting buttons means I can't add things like noinclude and nowiki tags which are commonly used by experienced editors on talk pages.
At the top of the new topic box, I see a link telling me I can switch back to "the legacy experience". Hallelujah, normal service is temporarily restored. At the top of this screen I see an invitation to "visit preferences to set the legacy experience as your default". I followed the link and saw no reference to the "legacy experience" anywhere. Eventually I guessed that the relevant option was "quick topic adding". Why not just tell us that, rather than using vague marketingspeak in one text and the actual name of the feature in the other?