Maybe not "officially", if such a bright line exists. But in practice, a "reminder" of a law including a legal citation and phrases such as "without undue delay" will be interpreted as a threat in many circles. Whether or not it will be seen as a credible threat is a function of the recipient's risk tolerance, prior experience, familiarity with legal processes, etc. Hiring a lawyer ($$$) isn't out of the question if the recipient is unsure or has a low risk tolerance (as, perhaps, an individual blogger might have). It is a little cruel for a researcher to assume every recipient will respond neutrally.

I've received legit legal threats over email from very serious people. I'm certain they would have escalated to more "official" channels if the situation wasn't resolved to their satisfaction (if you're curious, it was a contractor billing issue that was being ignored by my HOA. I was caught in the crossfire).

The fact that random scammers are calling folks claiming they're violating tax law isn't a justification for researchers to engage in similar acts. Do you agree?