Pretty much not an issue. I've yet to see the case where deficiencies were caused because dechlorinator was dosed.
There is quite a bit of marketing / propaganda going on, with Seachem saying that they don't understand how on Earth the product they sell does this. Magic perhaps, can't argue with that.
https://www.seachem.com/prime.php
Also worth pointing out the word with a clear meaning "remove" is absent, instead it "renders them non-toxic", "detoxifies" or "reduces the formation". This ability is often associated with overdosing and the companies do not provide any dose-response data, so something smells (like prime). Maybe it does detoxify / reduce / remove nitrate , but if 1x lowers 0.01 mg/L(ppm) NO3, does it matter ?
For heavy metals the issue is somewhat simpler, they use chelators like EDTA (ethylenediaminetetraacetic acid). Page 4 has the ingredients for another dechlorinator JBL Biotopol
https://www.jbl.de/?mod=productpdf&id=2316&country=us&lang=en . So take for example copper coming in from the pipes / hot water system, dose water conditioner with chelator and it becomes less toxic . But then again trace mixes like CSMB will also have chelated Cu and we know plants can use it despite it being 'detoxified'.
Making people afraid is a big source of profit. So it makes sense to claim to 'detoxify' everything.