man, this takes me back about a decade or so. Basically there was a group of people who discovered that making people afraid of an unknown, largely unmeasurable enemy makes them look interesting. Everything that was wrong was due to a micronutrient toxicity. They were never able to produce controlled data to back-up their claims, just random biased observations. Basically a non-issue, skip over and use the time to trim and clean your tank.
Anything can be toxic if at high enough concentrations. Ammonia is likely the first one aquarists hear of and yes, at high enough concentrations ammonia can even be toxic to plants. Copper is another one many people are familiar with, invertebrates being very sensitive to it but some Cu based medications go high enough to damage plants. This is often times many 10x or 100x more than would be dosed using the recommendations of EI.
Effect depends on the dose and the plants. Sometimes only less growth will be observed but can also go as far as necrotic spots or complete melting of the plant. These 'symptoms' can also come from something else and most often are not from toxicity but deficiency or other improper conditions.