I'm not even sure where to begin troubleshooting.
It doesn't work on a fresh save (command "/cheat all" to quickly get some splitters)? Do you have Alt-mode enabled to see the deconstruction planner filter appearing? Does the button on the side exist on when splitter UI is open? Is the mod enabled in mod manager?
Could I get a save file where it doesn't work?
Okay, so it's not working on the mainbus splitter because both output belts are connected. There's nothing to "block".
It blocks only when a splitter has one output and has no priorities set.
Look at the information page how the splitters have only one connected output belt.
Hmm....
Hmm....
Most likely I misunderstood the mod description...
I don't quite understand then, why block the unused divider strip if nothing will go there anyway?
Is it possible to add functionality where the user will be able to block the band being used?