It’s a plugin made by rkspsm, we discussed it on kde krita forum and he/she eventually put it together.
The plugin switches last brushes (default “/”) together with foreground/background color (default "x) and merges the command into one shortcut. There are some options to retain erase mode or not. It’s simple but someone might find it useful to speed up their workflow.
Just wanted to post it here, not sure if rkspsm knows about this forum.