In Rhythm of War, Navani mentions that perhaps Soulcasters, specifically Soulcaster metal, are another form of a Radiant spren. She uses the line ''Somehow the ancient spren had been coaxed into manifesting as Soulcasters instead of Blades?". Could this somehow be related to Testament and the brokenness of the Soulcaster Lin Davar and then Shallan had?
I'm not exactly sure what you mean by related. But just in case, Testament was not the Soulcaster. But the fact that spren become Soulcasters is related to this in some ways.