Rand Fishkin quietly added the word "feminist" to his twitter bio with sadly predictable results.
Twitter has been developing new tools and systems to protect its users from harassment but since its creation, it's been known as a launch pad for angry mobs and a generally horrible place to be if you ever take a strong, contradictory, or nuanced opinion. It's especially brutal for women and minorities, both in positions of power and activism. Many have credited the "140 characters or less" part with being the problem...have you ever read a discussion between two angry people on Twitter? Insults are easier to fit into 140 characters than the elaboration of your viewpoints. But earlier in the year, Twitter's own CEO painted the problem as being due to a lack of enforcement, weak tools, and company policy.
Here's an example of someone being harassed to the moon and back for their controversial opinion: Suey Park, famous for starting the #CancelColbert hashtag.
Things don't seem to have gotten much better since then. Perhaps Twitter's new tools and policies will create a positive change, but in response it only seems like the bad guys are becoming more persistent, angrier, and more organized.