I've presented corroborating evidence, in the form of the newspaper article. I'll admit it's not absolutely ironclad, but it would not be trivial to fake. Under the circumstances, it's more "official" evidence than I would expect from any other death of an American national living in Japan, all else being equal.
Meanwhile, in this thread, you've made several falsifiable claims that haven't been borne out. Starting with the claim that there is affirmative Japanese documentation that no Americans died in Japan during the relevant time period. Still waiting for you to back that one up. Then you claimed that the journalist didn't provide an official quote from Byuu's employer, which they clearly did. Now you're gone from claiming that we don't have sufficient evidence one way or the other to saying I should "be happy that Byuu is still alive". Where's your documentation for that claim?
You seem very invested in convincing other people on the internet that Byuu faked their death. I'm pretty sure that if I were able to produce a death certificate, that you would just move the goalposts again and say that there's no proof that David Ginder was really Byuu. (Or would you commit now to accepting a death certificate for David Ginder as proof that Byuu was cyberbullied into killing themselves?)
You do realize that obituaries don't just happen? Your loved ones (or your estate, etc) have to pay the newspaper to run them. If they don't, then nothing happens. There's no government agency out there making sure everybody gets an obituary.
Here's my question to you. Show me an example of the "documentation" you want for somebody, anybody, else. If it's public information that's so easy to find, I'm sure you'll have no trouble at all pointing to an example. After all, people die in Japan everyday.