they (like most suspension pieces) were rubber encased due to cost and making a softer ride and long life (as opposed to steel bushings and the like).
Polyurethane bushings (for what-ever application) didn't become popular until the racing community needed a bushing that didn't bind, but was very unforgiving when stressed.
As for the 928 rack bushings, I suspect the intent was to limit bumps and vibrations up through the steering mechanism into the hands of the driver.
Remember, the 928 was a grand touring car, not a race car. Big difference......
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