2nd sentence says "The number of elements shall be equal to the value of the valueRank field."
This should say "The number of dimensions shall be equal to the value of the valueRank field."

The paragraph would be more readable and make more sense by making it this:

"The number of dimensions shall be equal to the value of the valueRank field.
This field shall be null if valueRank ≤ 0, otherwise the value in each dimension specifies the maximum supported length of each dimension. If the maximum length of a dimension is unknown the value shall be 0.
Regardless of the number of dimensions, the maximum number of elements in an array transferred on the wire is 2147483647 (max Int32).
"

This would help eliminate confusion over the meaning of a non-zero value in a dimension. It should not be interpreted as a constant length, it is meant as a maximum length.

Question: are ValueRanks of 0, -2, and -3 valid? What is the use case for this?

If these are allowed then fields with this ValueRank will be extremely difficult for a client to handle all the edge cases, especially if it is allowed to change for each value.
IMO it should explicitly define a ValueRank of 0, -2 and -3 as illegal.