There's also a nice addendum on cstheory.stackexchange, "What's new in purely functional data structures since Okasaki?" - https://cstheory.stackexchange.com/questions/1539/whats-new-...
1. PaC-trees: Supporting Parallel and Compressed Purely-Functional Collections - 2022: This paper introduces PaC-trees, a purely functional data structure that supports parallel and compressed collections. PaC-trees are designed to be efficient in terms of space and time complexity while maintaining the benefits of functional data structures.
2. Proving tree algorithms for succinct data structures - 2019: This paper discusses the development of tree algorithms for succinct data structures, which are compact representations of data that support efficient query and update operations.
These is some work in other fields too:
1. CyBy2: a strongly typed, purely functional framework for chemical data management - 2019-12-30: This paper presents CyBy2, a purely functional framework for managing chemical data. The framework is designed to be strongly typed and enforce referential transparency through the type system.
2. chemf: A purely functional chemistry toolkit - 2012-12-20: This paper introduces chemf, a purely functional chemistry toolkit that provides a set of algorithms and data structures for working with chemical data in a functional programming context.
What's new in purely functional data structures since Okasaki? (2010) - https://news.ycombinator.com/item?id=11056704 - Feb 2016 (42 comments)
What's new in purely functional data structures since Okasaki? (2010) - https://news.ycombinator.com/item?id=7081191 - Jan 2014 (17 comments)
What's new in purely functional data structures since Okasaki? - https://news.ycombinator.com/item?id=1983461 - Dec 2010 (2 comments)
What's new in purely functional data structures since Okasaki - https://news.ycombinator.com/item?id=1713594 - Sept 2010 (1 comment)