For agencies supporting many sites, whose users have no way of knowing whether Gutenberg will break their site or not, this nag screen is a danger. Pre-emptively installing classic editor unfortunately won't suppress the nag notice either, but since classic editor is being used as a bellwether of the success of Gutenberg, it's important that you install it, if you expect issues.

I co-wrote a plugin that solves the issue. If you install Classic Editor and Classic Editor Addon, the nag screen will be suppressed, and when 5.0 releases, Gutenberg will be automatically suppressed as well.