Should I use 写的是"..." vs 写得是"..."? For example, if I want to say


  他写de是"..."


which "de" should I use?

是 isn't a noun, so that suggests using 得
but on the other hand, I've seen usages of "写的是什么" (on Google search at least)
There are many similar questions on this site.

I think we use 的 in this case.

Because
他写的是...
is a simplified form of
他写的内容是...

内容 is a noun, therefore we can use 的 here.