Maybe you haven't heard that word in this context before, it's of course not meant to be read literally. I understand it the same way as I would read "toxic workplace culture" or "toxic social environment", which is that there are people who bully, abuse power, take advantage of you, deliberately misunderstand you, are racist, sexist, etc.
1) containing or being poisonous material especially when capable of causing death or serious debilitation
2) exhibiting symptoms of infection or toxicosis
3) extremely harsh, malicious, or harmful
4) relating to or being an asset that has lost so much value that it cannot be sold on the market
In the context of this specific discussion, for the sake of disambiguation, the third definition listed above is the one being used. Here are some articles which may help to further clarify the concept of "toxic" as it applies to people, social interactions and online communities: