The term 'persistent data structure' goes back to papers by Tarjan, et al in the 80s but the path copying technique is much older. My favorite technique from one of those papers, the in-place v2 trick, is an alternative to path copying that achieves amortized O(1) space per update as opposed to O(depth) space per update for path copying; this only gives you partial persistence (linear history like an MVCC database system, not branching history like a purely functional data structure or Git) but often that's all you need. (There's also an extension of the trick to full persistence but it's not very practical.)

> I’m not a Clojure expert, but I believe you could trace a direct line to Clojure’s core immutable data structures from Okasaki’s doctoral thesis.

I believe the direct inspiration for Clojure's persistent vectors and maps are Bagwell's papers on HAMTs, which weren't so much a new data structure as an example of data structure engineering. Okasaki's thesis has several contributions but its main theme is how you can apply amortization in the purely functional setting if you have laziness or memoization, which is in a very different direction.