But clearly -- assuming adequate filtering is possible, it's not at all a necessary improvement, so don't sweat leaving it open, either. If I am using the shield, is the above drawing optimal? Or should I do separate shielding for any signal as well?

Understanding the Context

For example, [UART TX and RX in a shield] + [all remaining signals]. Sodium for example is readily ionized even at mild flame temperatures -- hence the ubiquitous orange sodium glow when placing things into a fire (there's inevitably traces of sodium on things, for various reasons: sweat, dust, water, etc.).