Death By YoYo started off with a delrin design, then later released an aluminum as well. I think it was shortly after the Milk came out. Machined delrin yoyos were fairly unexplored, so it became a notable characteristic of the company to start off that way. The company ended when the owner sold a bunch of yoyos and then never shipped them out, and then disappeared from the yoyo community.
I don’t know if Buzz-On ever formally ended, so there may not be a publicly known reason for why they disappeared. At one point, Dave Bazan tried to branch out into a new company called Throw_Yo. The failure of that company may have had something to do with the end of Buzz-On. I think he also moved from Texas to Idaho, which may have contributed. Someone who knows Dave personally might have more accurate info.
Throwdown was a short-lived company from roughly the same time period as DBYY. I think they disappeared after their first yoyo had a critical flaw and they could not afford to refund or replace the high rate of broken yoyos. Before that, they sold attachable weight rings for FHZs.
Furph Yo used to custom powder coat yoyos and then released their own yoyo called the Rok. It was poorly received, and they quickly stopped making yoyos.
I don’t know if these ever disbanded, but I haven’t heard from AnY, XYo, or HICOO in a while. All three are Asian, so if they ever announced anything about closing, it might have been missed in English-speaking communities. Or maybe there were announcements that I just missed. AnY was probably best known for the MiRoC, which had bearing caps on the side of the yoyo (the bearing caps didn’t work very well, though–I think some of them had problems staying attached while the yoyo was spinning). XYo was probably best known for the XCalibur, which I think may have had some sort of interesting coating. I never saw one in person, but I remember that being a point of interest. I’m curious if anyone knows what happened with those companies.