I’m just looking into the source code of the ExecutionContext class for CoreClr:
https://github.com/dotnet/coreclr/blob/775003a4c72f0acc37eab84628fcef541533ba4e/src/mscorlib/src/System/Threading/ExecutionContext.cs
Base on my understanding of the code, the async local values are fields/variables of each ExecutionContext instance. They are not based on ThreadLocal or any thread specific persisted data store.
To verify this, in my testing with thread pool threads, an instance left in async local value is not accessible when the same thread pool thread is reused, and the “left” instance’s destructor for cleaning up itself got called on next GC cycle, meaning the instance is GCed as expected.