I recently asked if anyone could point me to an (open source) app on github or wherever that has a 'fluid schema' on a NoSQL system and nobody was able to show me one. Do you have a concrete example I can look at please?

Nothing? For something as common as you say I'd have thought there would be many examples out there. I don't get how this can be touted so often when there's no concrete use cases for this 'feature'.