Is it safe to use raw mushrooms to flavor a broth?

In Norway lots of people still pick their own mushrooms and the Norwegian safety rules for mushrooms picked in the wild do not apply to Champignons you buy in a supermarket in 99.99999% of all cases. ¹

_{Note ¹: Unless something went horribly wrong in the food supply chain.}

I've found a source of some interesting background:

Mushrooms traded as food. Vol II sec. 1 Nordic Risk assessments and background on edible mushrooms, suitable for commercial marketing and background lists. For industry, trade and food inspection. Background information and guidance lists on mushrooms published by the Nordic Council of Ministers says on p.51:

As it is concluded from animal and in vitro studies that the phenylhydrazine derivatives occuring in Button Mushroom (A. bisporus) as well as the mushroom itself may be genotoxic and carcinogenic, a carcinogenic risk for humans cannot be excluded. It is therefore recommended not to eat Button Mushroom in larger amounts. A significantly higher intake than 2 kg/year (average consumption in Denmark, Iceland, Norway and Sweden) is regarded as “larger amounts”. Proper processing of the fresh mushroom reduces the amounts of potentially carcinogenic constituents. The fried, microwaveheated, boiled (especially if boiling water is discarded), and canned mushrooms contain significantly less of the potentially carcinogenic phenylhydrazines. Also ordinary freezing and subsequent thawing (but not freeze-drying) will reduce the content of phenylhydrazine in the mushroom. It is therefore recommended to process/ cook Button Mushroom before consumption

but note that this specifically states that frying reduces these compounds significantly. The precautionary advice is also based on consumption of >2kg/year. Similar advice is not given in many countries sharing common food regulations and cultures. So if you're only eating them occasionally, my interpretation of the quote above is that you've got nothing to worry about, especially is you're doing any kind of cooking.

Frying the mushrooms first is a real help if you want to maximise the savoury flavour from them, but pre-boiling is likely to extract some flavour which you then discard.

This might be the cause of those worries, on general mushrooms.

https://www.scientificamerican.com/article/raw-mushrooms-hazardous-or-harmless/

If you follow the link (I didn't post direct because I think Scientific American listing it makes it a bit more credible than some blog's opinion), the dietician states that, while there is agaratine, which is a toxin, in many mushrooms, including those people normally eat raw, the practical dose makes it a non-issue in many cases.

See also http://www.anh-usa.org/supermarket-mushrooms-dangerous-to-eat-raw/

If you want to be sure Google up "raw <your mushroom variety> recipe". But I'm not losing any sleep over this myself over the mushrooms I find in my supermarket.

Also, if you're flavoring a broth, it really doesn't hurt to heat the mushrooms if in doubt and heating might actually be better for the flavor aspect. If you this was about mushroom in salads or the like then my own opinion is that that level of concern is unwarranted.

Edit: to be clear - look up what's relevant to the specific type of mushroom you are interested in, not the recipe or mushrooms in general. Be safe.