We don’t have plans to remove the Markdown-only mode.
Initially, this will be controlled by a site setting, where admins can turn on the toggle to switch between rich text editor / Markdown-only editor, or leave it off to keep their composer Markdown-only. At this point, we do not have specifics on how both versions of the composer will be supported longterm (e.g., whether composer type will continue to be controlled by site setting, left up to user preference, or something else).