If you read their current terms - this actually does nothing to change their current policies, but rather clarifies what is not allowed. In fact, regarding their recent investigation of activities by pixiv users, "Some were found to violate the pixiv Terms of Use, Article 14 "Prohibited Conduct", Item 26."
This means there isn't a change coming - but a clarification of the current TOU. Just because few people got caught doing something against current TOU in the past doesn't mean it was okay to do so on that site. It means they weren't caught. Now that more are being caught - pixiv is clarifying for users to know what's not allowed.
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: When using the Services, User must not engage in the conduct set forth in the following items... Conduct that is linked to or likely to be linked to fraud or
other criminal behavior
Criminal behavior is legally understood as where a person resides. In many countries, what is soon to be clarified in their terms is considered criminal behavior to create, produce, and share. The company wants to stay in business as long as possible, and therefore chooses to limit their legal liability in order to do that business.
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: Using the Services to display, sell, purchase, register, or engage in other transactions involving the products set forth below...
4. Products that contain images or other data corresponding to obscenity
This is the Article the announcement points to and, as you can see, they are now being more clear about what "obscenity" means with regard to their TOU. What pixiv considers obscene includes, but is not limited to:
- Sexual exploitation of a minor
- Incest
- Bestiality
- Rape (or any other non-consensual sexual behavior)
- Non-consensual mutilation of a person or body part
Again, nothing has really changed regarding the TOU - it's just people got away with it for so long because of lack of proper enforcement (either due to staffing issues or lack of motivation or other reasons).