the question is essentially the title: are stdlib targets themselves built with the runtime exclusivity checks on or off? i've found a number of references in GitHub search that suggest at least some stdlib code is opted out of the dynamic exclusivity checks, but i don't really understand the build system enough to tell for certain.
As of today, I think the answer is still that the stdlib (and friends) are not built with runtime exclusivity checking enabled. Here is a semi-recent draft PR exploring the consequences of changing this. It seems the performance impacts of enabling it are still fairly substantial.