If you are using a block theme in WordPress, the customizer will be removed, unless you have other plugins etc. registering customizer items.
I quite like the “additional CSS” part of the customizer as you get a live preview of your changes.
Therefore, if you want to re-enable this, you can do so by adding this code to a plugin or your theme functions file.
add_action( 'customize_register', '__return_true' );