What is eduPerson and how do campuses use it?
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Why does eduPerson include the eduPersonOrcid attribute and not eduPersonResearcherId? Won't this lead to new attributes for every kind of identifier?
Yes, it will, very deliberately. Combining multiple types of data into a single attribute precludes use cases in which only a subset of that data may be relevant, unless the data is encoded in a way that allows the different types of data to be recovered. That in turn adds extra work to a consumer of the data.
Attributes are not "expensive" to create, and the more precise an attribute definition can be made, the more intelligent software can be when dealing with them. We should expect to see additional attributes created for any kind of identifier that gains adoption by the community.
How are Identity Providers and Service Providers leveraging the eduPersonOrcid attribute?
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Is eduPersonTargetedID really an attribute in the usual sense? What is the relatonship between it and SAML? Is it relevant in a context other than SAML?
The eduPersonTargetedID is an unusual attribute that does not map easily to an LDAP representation in the way that every other attribute in the schema does. Because its value is intended to be different for every "client", it cannot easily be maintained in a typical LDAP directory and is not expected to be. That indeed makes it unusual.
The relationship with SAML has to do with the history of Shibboleth and its use of eduPerson as the "recommended" attribute vocabulary for the higher education community's use of SAML. The concept of a "directed" (pair-wise) identifier emerged from the work done on federated identity when SAML 2.0 was being standardized, and because Shibboleth was originally a SAML 1.1-based system, the Shibboleth community decided to develop an eduPerson attribute that had the characteristics of a concept in SAML 2.0 called a "persistent name identifier". That attribute was eduPersonTargetedID.
As the use of SAML 2.0 supplanted SAML 1.1, the need for an attribute distinct from the already-defined SAML NameID Format of "urn:oasis:names:tc:SAML:2.0:nameid-format:persistent" has waned, and the need for something called eduPersonTargetedID is now somewhat historical.
As to other contexts, that is unclear. The definition of eduPersonTargetedID is suitably generalized to be compatible with the SAML concepts it was copying, but may or may not be suitable as a way of describing similar concepts in other standards. This is an open question.