This plugin 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.

And it’s on GitHub.

How is this plugin different from Classic Editor plugin?

By default, Classic Editor allows individual users to select whether they want to use block editor or classic editor on their profile edit screens. This feature must be disabled on Settings > Writing screen.

Additionally, Classic Editor stores post meta data in the database about type of editor that post has used.

On the other hand, Disable Block Editor disables block editor for everyone and everywhere, without any options or settings to configure, and without storing any data into the database.

How is this plugin different from Disable Gutenberg plugin?

Disable Gutenberg allows much finer control on where to and for whom to show block editor, and that is achieved by configuring settings. Basically, same differences as with Classic Editor.