For agencies and developers with multiple clients, these experiences this "late" in the game are deeply concerning. I put together a helper plugin and workflow for my agency to get Classic Editor installed on all our 400+ client sites before the merge proposal or the 4.9.6 release, to strip Gutenberg and the relevant nag panel until we can fully test integrations with the final release version, and set client expectations.

You can find my solution here: https://gschoppe.com/wordpress/disable-gutenberg-now/