Sites where threads about yukkuris are common:
Threads about yukkuris are created on the /jp/ board in 4chan, (link here), and have become a fixture on the site. Typically they each last several days, and when they reach the image limit(100 images) or stop being bumped to the top of the board(200 posts? Not sure), a new thread is created and linked to from the old thread. Attempts to sabotage the thread by "saging" it to bring it down are common, however they are generally ineffective. While /jp/ is a worksafe board, yukkuri sex and guro pics seem to be tolerated by the board mods because it doesn't resemble human sex or guro.
Yukkuri Threads are also created on Pooshlmer, (link here). On Pooshlmer, the custom is to create separate threads for abuse and non-abusive yukkuri content, called "non-Family Friendly" and "Family-Friendly" threads respectively. The non-family friendly threads, however, are often disrupted by family friendly trolls. Indeed, several months ago there was an incident which lasted several days in which pooshlmer was dominated by threads attacking abuse (and the members who liked it)- it largely died down when the admin declared yukkuri abuse safe, and deleted the NSFW content that anti-abusers were using to justify the deletion of yukkuri abuse (e.g. guro). It continued, although at a lower intensity, for several weeks afterwards with several members questioning the authority of the admin over the issue.
On Futaba chan, yukkuri threads may appear on any of the "2D" boards on the site, and they frequently contain edits and guro. There seem to be attempts at separate threads for abuse and non-abuse, but are not quite as rigorously enforced as on Pooshlmer. Futaba 2D boards with board name and server name: 二次元 (tmp.), 二次元裏(jun.), 二次元裏(may.) 二次元IDnew (may.)