It's nice to have a range of causes to troubleshoot.Its interesting to read both articles bc they give very different reasons why soft shells occurs.
These links also offer alternate causes for calcium deposits on eggs, which I've been wondering about, discussed briefly last month on this thread after a post by @Perris that cited excess vitamin D.
However, around here, only Miss Hazel (Cream Legbar) lays eggs with calcium deposits, and has for 4 years, so I can assume the issue isn't diet-wide.
Deposits are worse when Hazel's extra excited about oyster shells the day before, like when I add new shells to their cups.
I could try to limit her intake but am loathe to keep her from something she's so passionate about. It's possible that, because she pops out such round eggs for her small frame, she needs the oyster shells for help with laying contractions.
Or she wants more calcium for her bodacious spurs; check those out!