IQ is an ordinal score in that it’s relationship to outcomes of interest is nonlinear, but for the most important outcomes of interest, e.g. ability to solve difficult problems or income or similar, the relationship between IQ and success at the outcome is exponential, so you’d be seeing accelerating returns for a while.
Presumably fundamental physics limits how far these exponential returns can go, but we seem quite far from those limits (e.g. we haven’t even solved aging yet).
IQ tests are built on item response theory, where people’s IQ is measured in terms of how difficult tasks they can solve. The difficulty of tasks is determined by how many people can solve them, so there is an ordinal element to that, but by splitting the tasks off you could in principle measure IQ levels quite high, I think.