We understand that this will need some adjustments to get merged - but I hope you'll work with us on what to improve - so we can get this feature merged to the benefit of everyone :)

Did not look into much code but I think only adding one label may be not enough. And adding n labels per column needs a different backend structure.

we add 1 label per column - and remove "the other column labels" - when you switch issue to new column. That way you can search for ALL issues in a certain column (like 'ready' - or what states you want to use for your kanban columns)

We use this for several things - and also the fact that we now get notifications - means our webhook informs when someone moves issues around in kanban

I understand that, but other people may want to add two labels when moving an item. And that would need a totaly different backend structure. Just don't want to see code for just one label merged just to get replaced some weeks later with code for multiple labels.

Having 2 labels for one state (column) - would seem a very odd requirement. Building the generic "do anything" - someone else did a PR for - but we were not able to make it work - so we opted for the simpler option that IMHO would fit the needs of many.
Later - if someone has the skills and time - I do agree that the "custom" - do any action on kanban column switch feature - should be developed.. But that does not remove the value of the "simple and enough for most usecases" option. - which will be simpler to enable.

would seem a very odd requirement

Column Testing could add test/ui and test/backend to trigger two different groups of people to do something. I don't think that's an odd requirement.