Suggestion from a translator

Hi,
I never tried your plugin, I just had a look on some of the strings for translation. Please avoid splitting up (and then joining back in the code) phrases. When you (try to) translate, you may miss the context. And some languages prefer to put the words in a totally different order than English.

There may be more places, but one that really caught my eye is the following (where you should simply put everything in one longer string!):

$msg  = '<strong>'. DISABLE_GUTENBERG_NAME .'</strong> '. esc_html__('requires WordPress ', 'disable-gutenberg') . DISABLE_GUTENBERG_REQUIRE;
$msg .= esc_html__(' or higher, and has been deactivated! ', 'disable-gutenberg');
$msg .= esc_html__('Please return to the', 'disable-gutenberg') .' <a href="'. admin_url() .'">';
$msg .= esc_html__('WP Admin Area', 'disable-gutenberg') .'</a> '. esc_html__('to upgrade WordPress and try again.', 'disable-gutenberg');

Use printf to replace a %s with '<strong>'. DISABLE_GUTENBERG_NAME .'</strong>'