Hey, thanks for reaching out & I'll gladly help you out, but in order to do so, we are going to need a bit more information from you, if possible.
Would you please send us your current 'Custom CSS' setup, so that we can test it out on our end and send you back the right adjustments? And which way would you like to show your related posts in mobile view?
As for your inquiry — yes, we removed that 'Mobile settings' submenu, since we now made all of our themes responsive, which should make your plugin settings page less clustered.
Let me know what's your current CSS customization and I'll be more than glad to help you out! Looking forward to your reply!