As far as I know, Mesa has never had a static-libs USE flag (and never will).
We have a number of superfluous [static-libs(+)] use dependencies in DEPEND/RDEPEND. This is by choice, otherwise updating this list for every dependency is an unmaintainable mess. Granted, the whole USE=static-user and USE=static is rather experimental. But we have some genuine use cases (and actual users). And the current approach makes sure that we support it "as much as possible".