Basically nobody in Japan has ever heard of this. Whenever I'm hanging out in Japan and run across another viral post about "The ancient and time-honored Japanese tradition of goldjoin", I ask Japanese people about it in Japanese*

Not once has anyone heard of it.

It's just another fake viral sensation that we cooked up in the USA

There's plenty of historical evidence that it's been done, but it's not some common thing that people in Japan regularly do. Modern Japanese people actually love buying disposable plastic crap, then throwing it out when there's the slightest thing wrong with it.

Since they're right next door to China, they can get a lot of fairly nice plastic stuff at the dollar store. You pop into the 100 yen store, and pay 100 yen +tax for the exact same plastic junk that would cost you $5 to $20 per item at Target or Wal-Mart in the USA

* For the record, I'm not asking "What's kintsugi?", I'm asking "Hey, have you ever heard of this thing where you fix a broken item using gold? Like gold glue?", and I show them the viral post du jour