WordPress 5 upgrade: until when is it safe to keep project running in 4.x?

As stated here:

The only current officially supported version is WordPress 4.9.8.
Previous major releases before this may or may not get security
updates as serious exploits are discovered.

And it has always been that way – only one version is officially supported.

This means it’s always a bad idea to postpone updates.

So if you’re afraid of Gutenberg, then you should:

1. Prepare a test version for your site and check if it will cause any problems with 5.0.

2. Disable Gutenberg, if you don’t want to use it.

You can disable Gutenberg with this code:

if (version_compare($GLOBALS['wp_version'], '5.0-beta', '>')) {
    // WP > 5 beta
    add_filter('use_block_editor_for_post_type', '__return_false', 100);
} else {
    // WP < 5 beta
    add_filter('gutenberg_can_edit_post_type', '__return_false');
}

or using one of available plugins.