Yes, some animals (generally predators) have a critical period for developing sight. Unlike language, this is an easy faculty to test in animals. One could try this drug on cats that had been blindfolded during the critical period (a standard experiment).
Experiments of this general sort have already been done. I haven't read deeply yet, but exploring the biblio it appears that the novelty here is that something that worked in animal models by a semi-well-understood mechanism was demonstrated to also work in humans via mere oral administration of an already FDA-approved substance.
...We found that chronic intraperitoneal administration of valproic acid or sodium butyrate (two different
NPR reports on a study giving volprioc acid to adults and training them on pitch (singing):
Brain plasticity is useful for a whole lot more than learning pitch. As the article notes it would be invaluable for training one's ear to pick up sounds of foreign languages, but also it seems reasonable to this commentator that high levels of plasticity during rationality training or other forms of self-development would result in more transformative results.