This data-block was originally added in eb4e3bbe68.
However, that original plan wasn't fully implemented, with simulations
now integrated with geometry nodes and modifiers instead of a separate
data-block. We kept the data-block around anyway since we have the
loose plan of using a similar data-block to make global simulations
connected between multiple objects. But it may be a while before we
implement that, and in the meantime having this just causes confusion.
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