The better attribute to use is eduPersonTargetedID.
We have found that a lot of Service Providers are requesting auEduPersonSharedToken for purposes other than its intended purpose. This is problematic because auEduPersonSharedToken, by design, does not preserve a user's privacy between services.
We're currently trialling an approach where only AAF staff can add this attribute to services who justifiably need it. We hope to encourage services to use the more suitable eduPersonTargetedID attribute, which is the preferred unique identifier.
If your service requires the use of auEduPersonSharedToken, please contact firstname.lastname@example.org and we'll review your use case and help you add this attribute to your Service Provider.