I might have a nonstandard interpretation of the word "mustn't". I have generally assumed that it means roughly "oughtn't" or "shouldn't", instead of fitting in any place where "must not" would go.

I don't know if my understanding is the common one, or if I just made up a rule by mistake that other people don't use.

Your understanding is the same as mine, and I had to read that sentence a couple times to figure out that "musn't" should really read "must not". So far as I know, "musn't" only means "should not" and is not a general replacement for all instances of "must not".

My intuition is different to yours: "mustn't" made perfectly good sense to me in that context. Just a data point.

The usage is correct, but very rare; I observe it more among non-native speakers, who generally are less intimately familiar with the farrago of special cases that constitutes so much of English usage rules in practice, including that which has "mustn't" by convention only used in the imperative mood.

What is all this? As a native English speaker, I view mustn't as a contraction of must not; other examples include cannot and can't, should not and shouldn't, and do not and don't.

So the usage of mustn't should be exactly the usage of must not because that's literally what it is.

"You must not spend time on US conspiracy forums."

"Must" is an imperative or a command. "Should" is a recommendation.

Long time after the fact, but this was my understanding of contractions also, and was my intended usage of "mustn't". English is a crazy language, especially when we all seem to have been shipped with different compilers.

I mean you're not wrong in the narrowly denotative sense, but there's a distinction to be made between the way words are defined and the way they're conventionally used. I'm describing my observation of the latter. (So are you! But you observe differently.)

I think I am unusually permissive about where I allow contractions. I'm always tickled by that E E Cummings poem that just barely pulls off a rhyme with "year" and "appear" in its last line, "...and April's where we're.".

In this case one suspects OP wanted both meanings...

Not that witty :P