Some people believe discarding iron items e.g. screws in the soil "make is rich in iron, for the good of the plants". This is in the context of the dark soil in northern Bulgaria. Safety concerns aside, is there any truth in this folk saying?
This adding nails or screws to soil to increase iron content is a myth because any iron that's given out is in a form that's inaccessible to plants, see here https://laidbackgardener.blog/2018/09/08/the-rusty-nail-myth-2/
Add to that the fact that many modern nails and screws are not iron anyway,and it's even more pointless. Iron in a chelated form can be useful to plants, if there is a shortage in the soil, though iron shortage is not that common.
I don't know if it makes sense in your situation, but iron is an essential element for plants. The chloroplasts in the leaves need iron for example for cytochrome, an important component in the photosynthesis process.
However, like with all compounds, too much of iron can become toxic. In soil more factors are involved which will make iron available or not (think of e.g., pH and organic matter content, and many more factors). Iron in rust is Fe3+, but plants want to uptake Fe2+. In soil this redox reaction can occur with lower pH, see here a scientific paper about Iron in soil.
For your situation in Bulgaria, I don't know. I don't know if your soil is deficient of iron, or not. Do you have iron measurements from your soil? Also do you have other properties (pH, organic matter content) of your soil available?
The premise of Bamboo's link is that iron needs to be soluble to be available to plants. I'm not sure how true that is, but it's a thought I've heard expressed a number of times, a number of places, without any real evidence. I would concur that water soluble minerals generally seem to be more available, however. Bamboo does know a lot about plants, and is highly educated, however; so, I wouldn't take her source lightly.
Even if it's not available to plants at all, the oxide from the iron could potentially get displaced by something else (and may more likely do so in an acidic soil).
Iron oxide nanoparticles are apparently usable by plants, although I'm not sure it's wise to give them to them.
The following link lets us know that iron-absorption isn't well-understood (so, I'm guessing the true answer to your question is unknown, and requires further study to determine): https://www.researchgate.net/publication/335438161_Iron_oxide-humic_acid_coprecipitates_as_iron_source_for_cucumber_plants
However, in the researchgate link above, it does mention how plants used iron from a couple substances that contain related compounds to iron oxide.
I guess the way to figure it out is to get some iron nails, a soil devoid of iron, and transplant something into it. See if it gets iron-deficiency, and compare to a control in the same iron-deficient soil without iron nails. Of course, you'd want a thorough soil test on the soil, and make sure that the exact same soil can be replicated.
I wonder if humic acid (from compost) reacts with iron oxide, and if soil bacteria do stuff to it.
Plant roots can produce malic acid, citric acid, malonic acid (as well as possibly succinic, tartaric, piscidic, aconitic and fumaric acids; which acids they produce probably depends on the plant species and other factors) and can be part of the process in producing carbonic acid. So, if any of those acids react with iron oxide (and I'm not sure that any of them do), then plants might be able to use it, somewhat.
Apparently, soils can contain sugars, too, which means additional acids might be present in the soil. For instance, I hypothesize that some sugars might be converted into alcohols. I know that alcohol can be converted into acetic acid by acetic acid bacteria. I believe acetic acid has some kind of reaction with iron oxide, but I don't know what that reaction is, exactly. I'm sure there are loads of other possible acids and reactions.