This question was partially inspired by the show, but I knew about this before. Some cites on salt. If you consider (semi-)historicity a spoiler related to what will likely happen in the future of Ragnar et al., those are in there FYI:
[1] Skeptical, but not outright doubting the practice.\ (just found this, note person’s comment on salt being symbolic if anything)
In any case I am not asking directly about the historicity of whether it occurred (although you are free to debate it) but whether the salt has any practical use or whether it was symbolic and why it was mentioned.
Your first source refers to late 18th, early 19th century historians. Maybe they used a more contemporary source to make their claim about the detail of the salt, but so far I haven’t seen one.
The second source could very well be using the same modern historians as their “one source”.
I got, and get, the intent of your OP, but without some emphasis on the lack of evidence for the practice, this thread will work to increase ignorance by making people remember it as a known part of the blood eagle punishment.
You appear to think we know a lot more about Norse spiritual beliefs than we actually do. What we have are hints and fragments, many of them filtered through being recorded by Christians with their ideas of afterlives as either reward or punishment.
The wikipedia article has three sources that directly mention it. The saga litterature was put on paper in the 13th century, but is generally considered somewhat accurate. There is also a contemporary source that covers one of the same events, but doesn’t include the blood eagle.
That’s the pattern for sources about the Norse period in general. There are no contemporary sources written by the Norse themselves, and only short and in the passing mentions in contemporary sources from neighbouring Christian states.