it's always a little annoying to hit someone with a disk or something that should kill them but it does nothing for some reason..? then you see it was because you didn't notice they had stubborn lol

(this has happened so much to me, catching someone out of a jump with a cube for example and they survive where they should have died)

Just a simple effect like Adamant has would be a really nice thing as there isn't really any way to know if their stubborn is up or not other then if you noticed they have stubborn on and saw that they dashed before you hit them

Yeah, I think some blue particles while stubborn's effect is active could be a nice addition to communicate precisely when the effect is active. This might be useful for both the player using stubborn and the player fighting against an opponent using it.

Additionally, perhaps there could be a "ding" sound or something similar when a structure hits the player with stubborn stone active?

Yes I just had this idea and went to make a suggestion for it, but found yours

It should have a dark blue effect, that adds on to flow-dash jump or overrides it. It'll be cool to see it activate on ground dashes too, nice to give players more individuality, and it'll pair nicely with the future charge stone update