[robespierre]

It turns out that the only key that will need retrobright is the space bar!

That is usually the case, since the other keys are typically molded in PBT (polybutylene terephthalate), while the space bar is ABS. ABS is the main resin that suffers yellowing, for reasons that remain unclear.
The space bar is ABS because its length presents greater loading forces and requires extra toughness. A space bar in PBT might crack.