Thanks for linking to the original source. The original source appears quite dubious, for reasons detailed below.

The eye-catching graph reproduced on the LinkedIn post is Figure 2 in this paper you linked, "Challenges in the Diagnosis of Magnesium Status." It is labeled as "The average mineral content of calcium, magnesium, and iron in cabbage, lettuce, tomatoes, and spinach." Note that 3 of the 7 data points -- including the two highest points -- are marked with asterisks as "numbers could not be independently verified." Excluding the asterisked points, there is one measurement in 1948 at around 200 mg and then 3 measurements in 2000, 2004, and 2018 all of which look similarly low -- I eyeball them as around 25 mg.

The oldest non-asterisked value is from 1948, cited as

Firman Bear. Ash and Mineral Cation Content of Vegetables. Soil Sci. Soc. Am. Proc. 1948;13:380–384.

The actual paper is here:

https://dl.sciencesocieties.org/publications/sssaj/abstracts...

Sadly, it appears to be in a journal old/obscure enough that sci-hub does not properly retrieve the full text of the article.

The results appear to be from a report commonly cited as the "Bear Report," numbers reproduced here:

https://njaes.rutgers.edu/pubs/bear-report/phosphorus.php

https://njaes.rutgers.edu/pubs/bear-report/ash.php

(See also https://njaes.rutgers.edu/pubs/bear-report/ for a top level overview of the report.)

What is the value plotted in Figure 2? It does not correspond to the sum of iron, calcium, and magnesium across any of {cabbage, lettuce, tomatoes, spinach}. Taking a guess, it looks closest to the highest reported value for spinach in magnesium (203.9 mg in the Bear report). Note however that the lowest value for magnesium in spinach in the Bear report is 46.9 mg. Whatever is being plotted here, it's also not an average.

Now take a look at the citation for the modern numbers in spinach:

https://ndb.nal.usda.gov/ndb/foods/show/11457?fgcd=&manu=&fo...

It reports an average of 79 mg of magnesium, 99 mg of calcium, and 2.71 mg of iron. There is no single nutrient number or sum of numbers that makes sense as the value plotted in Figure 2 of "Challenges in the Diagnosis of Magnesium Status." Figure 2 shows a number that I eyeball as around 25 mg for the modern (2000 and later) measurements.

The eye-catching claim of "estimates that the mineral content of vegetables has declined by as much as 80–90% in the last 100 years", buttressed by Figure 2 and apparently valid citations, seems to fall apart once you actually look at the citations.

Why would authors put so much effort into making such a confused, thinly supported claim?

2 out of 3 authors, including the first author, are employees of the Balchem Corporation. Their paper helpfully suggests that low serum magnesium levels "could warrant a medication change or dietary recommendations to increase intake of raw vegetables with higher magnesium content and reducing soda and processed food consumption with low or no magnesium and/or recommending magnesium supplements." (My emphasis.)

Balchem Corporation sells supplements to supply magnesium, calcium, and iron:

https://www.balchem.com/our-products/

What about the third author, Robert P. Doyle? He is an apparently legitimate professor at Syracuse University. Here's his faculty page:

http://thecollege.syr.edu/people/faculty/pages/chem/Doyle-Ro...

He links to this full list of his publications from his faculty page:

http://thecollege.syr.edu/people/faculty/pages/chem/Doyle-Ro...

This paper, "Challenges in the Diagnosis of Magnesium Status," is not listed among them. In fact, scanning over the titles of his listed papers, I don't see anything mentioning magnesium, nutrition, or soil in his publication history. Did the Balchem authors get some preliminary involvement from an actual professor and then add his name to the paper to confer legitimacy? Did the professor consider the final result so low-quality that he didn't want it listed even in his "full" publication list?

This paper has many problems that you can see just by trying to reconcile its Figure 2 with its corresponding citations, and other circumstantially suspicious factors. I would not put much stock in it.