I don't know the operative law, regulation, or procedure here. Though gag orders are certainly imposed with some regularity in investigations and legal proceedings.

IANAL but if the executive (e.g. police) can charge someone with obstruction of justice, i'd imagine they could reasonably issue an unofficial gag order irrespective so long as there is something under investigation..

So, there's another similar case mentioned here, but related to a Reason comment: with that one the government went and got a gag order from a judge.

In this case, they just sent that "preservation letter" and in the letter said not to alert the user: Techdirt (somewhat reasonably) is portraying that as "effectively" a gag order. They didn't go on even to issue a subpoena here, and so they also didn't go get an actual gag order, but it's something they've been known to do.