Graham McGregor, I get the feeling the project does not have anybody working on Blue Ocean development anymore. My guess is that whoever was in charge of it has left the company or moved on to other projects because I haven't seen any real improvements done to it in years. It's a shame because Blue Ocean was a GAME CHANGER!!! I personally love how Blue Ocean displays my pipeline and helps me quickly figure out an issue when something goes wrong. However, it seems to have issues that don't aren't being addressed and missing features (like this one) that could make it much. (In addition, Blue Ocean takes a very long time to load!)
I would LOVE first-class support for multiple levels of parallelism (such as nested parallel commands or nested matrixes in the declarative pipelines). Having that would let me load-balance and greatly reduce the duration of my pipelines. However, it just doesn't seem like a priority for anybody who has the skills to do anything about it.
I don't blame anybody working on Jenkins. In fact, I love everything they do regardless if they address stuff that make my life better or someone else's. No, I just wish my own skills included what was needed to help out in the same way the Jenkins has helped me.