Thank you for your question.
I want to thank you in particular for citing your source. This really helps us out. So many times, we have to guess: Is this a textbook question? Is this from an English translation of a Chinese translation of a Spanish translation of a Russian novel? Is this a quote from a semi-literate blogger? Is this person just making up these phrases?
By providing this information, you save us enormous amounts of time and energy. Again, thank you.
However, when you have several example sentences that you want us to comment on, please give them index numbers for us to refer to. This also really helps us. For example:
1: It is difficult to know what idioms are in fashion and what are not.
2: It is difficult to know which idioms are in fashion and which are not.
3: It is difficult to know what idioms are in fashion and which are not.
(I have filled in a period at the end of (1) which I believe you intended to be there.)
In the link you provide, Mr Woodham says that "what" and "which" are completely interchangeable in your examples. I don't entirely agree. While I agree that all three are both grammatical and comprehensible, they don't all sound as good.
Sentence (2) is by far the most preferable. (3) is somewhat acceptable but sounds strange.
I would not expect anyone with a proper third grade education to say (1).
This one doesn't work at all:
4: It is difficult to know which idioms are in fashion and what are not.
Mr Woodham gives more examples where he says that "which" and "what" are interchangeable, so all of these are, in his opinion, equally acceptable:
5: (Which/what) would you say are the most polluted cities in the world?
6: Do you know (which/what) sort of plants grow best in a shady garden?
7: I've no idea (which/what) road to take to Jimmy's place.
For (5), "which" definitely sounds better. I would only use "what" when everything else is singular:
What would you say is the most polluted city in the world?
Of course, (6) is ungrammatical either way. It should either be
6a: Do you know what sort of plants grows best in a shady garden?
6b: Do you know what sorts of plants grow best in a shady garden?
Either way, "which" is out of the question, unless the person is being presented with a list to choose from.
Similarly, "which" only works in (7) if it's a multiple choice question with only two or three possible answers. Otherwise, say "what".