The most likely explanation is that the OSAtomic.h header has been included with
OSATOMIC_USE_INLINED=1 defined, which causes the deprecation warning to be suppressed and expands the legacy OSAtomic operations with their <stdatomic.h> replacements inline. Note that this is intended as a transition assistant; C users should still plan to migrate to the <stdatomic.h> bindings, and Swift users should plan to migrate to whatever Swift settles on for atomics.
Quoting the OSAtomic header:
* These are deprecated legacy interfaces for atomic operations.
* The C11 interfaces in <stdatomic.h> resp. C++11 interfaces in <atomic>
* should be used instead.
* Define OSATOMIC_USE_INLINED=1 to get inline implementations of these
* interfaces in terms of the <stdatomic.h> resp. <atomic> primitives.
* This is intended as a transition convenience, direct use of those primitives
* is preferred.