The Sierra Club has a ratings system based grade and elevation gain. There’s also another system that I can’t remember the specifics of right now. But I have yet to see either one in use anywhere. Mostly it’s just informal descriptions. At least outside a few situations where parks have their add hock systems, but it’s extremely rare from what I’ve seen.
You could easily create a tagging scheme for trail difficulty based on the Sierra Club’s classifications though and it would be totally objective and verifiable since it’s based on real world, measurable criteria. I think it would work well in the United States if not other places.