Disable Block Editor is a WordPress plugin (download) that automatically disables default block editor for all post types and all users, without any settings or options.
It is very lightweight, just activate it and it works immediately.
Disable Block Editor is released under the terms of GNU General Public License. It is completely free so if you find it useful you can give something back to author. Your help could be done by giving donation, sending pull request on GitHub, sending bugs and suggestions, promoting plugin in blogs, forums, social networks, and other websites.
Note: plugin author is not obliged to provide free support via this site, email, or official forum. If you ask for free support, please be aware that it might skipped.