Hence it is stated that it is deprecated, it is still used by Swift S3 middleware or Nova EC2 emulation:

Auth is perfomed here:

• https://github.com/openstack/keystone/blob/62cd34d40d85af85b34146f9827da3414c09418f/keystone/api/_shared/EC2_S3_Resource.py#L39
• https://github.com/openstack/keystone/blob/62cd34d40d85af85b34146f9827da3414c09418f/keystone/api/ec2tokens.py#L36
• https://github.com/openstack/python-keystoneclient/blob/526350eaeeadef8c1ca5f80547c01f94342a6653/keystoneclient/contrib/ec2/utils.py#L80..L98

An example of the ec2 auth request: https://docs.openstack.org/api-ref/identity/v2-ext/?expanded=ec2-authentication-detail#ec2-authentication