If there is any branch of Western philosophy that is considered "anti-identitarian" as you name it, then it would rely on mereology which is the study of parts to wholes and systems thinking like it's practiced by the Santa Fe Institute. In essence, no object is a single thing, and all things are really composites. There have been influential thinkers in the West such as:
- Heraclitus who emphasized in the constant changing nature of things from opposite to opposite emphasizing the illusory nature of identity.
- Georg Hegel with his take on the dialectic which stated that the thesis-antithesis interplay leading to synthesis means that things are created by opposites and then clash with their opposites.
- Ludwig Beralanffy's General System Theory which emphasized that all things are systems and started the move towards modern systems thinking in terms of math and science.
In the East, it should be noted that Zen Buddhism more strongly rejects identity by rejecting duality. It is the case that Zen practitioners often use language in strange way to demonstrate the contradictory nature of labels and identity.
Accordingly, Zen demands that the practitioner overcome the dualism operative in the everyday standpoint, which it speaks of by using the phrase “not two.” The use of the phrase “not two” expresses Zen’s proclivity to favor the simple and the concrete, such that it is not expressed as a negation of dualism. This overcoming is an existential, practical project, a goal for the Zen practitioner, although it is paradoxically stated as “if you face it, it goes away.” This is because “facing” presupposes a dualistic stance.
Fascinating is the concept of the non-concept mu.
A monk asked Master Chao-chou, "Has a dog the Buddha Nature or not?" Chao-chou said, "Mu!
The general problem with fully rejecting identity is that we use it naturally when we think and reason. Definitions and explications in language are identities. Equations and congruences in math are identities. The entire analytical philosophical project is to recognize the value of language and science in using identities and equivalences to determine truth. As such, the moment one says I am an "anti-identitarian" one has engaged in the contradiction by identifying!