Deictics are also called demonstratives and in a broader sense also sometimes taken to comprise both indexicals and demonstratives. Indexicals are expressions that need to be interpreted by taking into account the utterance situation and deictic center I, here, now. Demonstratives additionally need to be interpreted with respect to an accompanying point gesture. German diese is plural of dies, meaning roughly these. It can be used demonstratively, as in "Welche hätten sie denn gerne? - Diese (+pointing gesture)." (Which ones would you like? - These+pointing gesture.) Da is not a demonstrative on its own, it can be a conjunction meaning because or as. It has a deictic use as sentence adverb as in "Er ist da" (He's here/He has arrived.) and a use as particle where it often combines with demonstratives and then roughly means near but not close the speaker, e.g. the answer to the previous question could be "Diese da (+pointing gesture)".
Agamben probably has the text-deictic use of such expressions in mind, which structure narratives. For example, in the phrases "as I have shown there already" and "here we have a nice example of", the indexical "here" and demonstrative "there" are used text-deictically, referring to some previous discourse.
which he argues do not refer to anything beyond themselves but only to their own utterance
If that's what he says then he's completely wrong. Deictics are communicative shortcuts which always refer to something else than their own utterance. The key difference to ordinary expressions is that you need to take into account aspects of their own utterance - who said what when in which way?, i.e. the deictic center and pointing 'gestures' understood in a very broad sense - in order to fully grasp what they refer to. This is not self-referentiality. It has been discussed under the label token-reflexivity, where a token is a physical linguistic sign. (For completeness, it needs to be mentioned that most contemporary accounts of the semantics of indexicals and demonstratives are not based on token-reflexivity but on David Kaplan's Logic of Demonstratives which is based on a two-layered model where linguistic meaning+ context= content and content+ circumstances of evaluation= extension.)
The term shifters has sometimes been used synonymously to deictics, e.g. by Jespersen. There is nowadays also a use of the term as shortcut for "context-shifting indexicals", which is a special linguistic phenomenon where indexicals shift their deictic center from I, here, now to some center of a reported speech act. This occurs in languages like Amharic and Zazaki. However, it is unlikely that the author has this special use in mind. He probably just means "Da and Diese as deictics".