I really liked that interface and haven't really found anything near as useable yet. Perspective JS (a free library by JP Morgan, you can try it here: https://perspective.finos.org/block/) has some really cool functionality in this direction but it has its one data processing engine, query language, rendering engine etc so we couldn't have used that for this project.
But, this is an interesting thought - I definitely would want to see this functionality in Pretzel - the only question is how to prioritize this. If you know of any performant table libraries that support collapsible pivots out-of-the-box, I'd love to integrate that. Alternatively, we'll write our own!