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I think you may be exaggerating slightly the peculiarities of legal English. The normal phrasing is, for example "This complaint is about a folder, which folder went missing on Tuesday last..." So which is natural and normal; it's folder that is unnecessary padding/zealous for clarity, as you prefer. The difference from your example is the difference between 'the folder which went missing' and 'the folder, which went missing,' ; minute, but possibly important.

Edit: the answer to your edited question is 'None directly, but it is a good habit to get into (for legal documents only).' Consider The folder in the principal's office, which went missing.... Strictly speaking, that means the office went missing, so which folder is required. (No reasonable person would be in doubt, but laws and lawyers, as Jay pointed out, can't rely on being reasonable.) In Jay's example, omitting a word could cost a lot of money; in mine it's almost certainly unimportant; in yours it makes no difference. So it's easier and safer to get into the habit of putting the extra word in every time.

I think you may be exaggerating slightly the peculiarities of legal English. The normal phrasing is, for example "This complaint is about a folder, which folder went missing on Tuesday last..." So which is natural and normal; it's folder that is unnecessary padding/zealous for clarity, as you prefer. The difference from your example is the difference between 'the folder which went missing' and 'the folder, which went missing,' ; minute, but possibly important.

I think you may be exaggerating slightly the peculiarities of legal English. The normal phrasing is, for example "This complaint is about a folder, which folder went missing on Tuesday last..." So which is natural and normal; it's folder that is unnecessary padding/zealous for clarity, as you prefer. The difference from your example is the difference between 'the folder which went missing' and 'the folder, which went missing,' ; minute, but possibly important.

Edit: the answer to your edited question is 'None directly, but it is a good habit to get into (for legal documents only).' Consider The folder in the principal's office, which went missing.... Strictly speaking, that means the office went missing, so which folder is required. (No reasonable person would be in doubt, but laws and lawyers, as Jay pointed out, can't rely on being reasonable.) In Jay's example, omitting a word could cost a lot of money; in mine it's almost certainly unimportant; in yours it makes no difference. So it's easier and safer to get into the habit of putting the extra word in every time.

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I think you may be exaggerating slightly the peculiarities of legal English. The normal phrasing is, for example "This complaint is about a folder, which folder went missing on Tuesday last..." So which is natural and normal; it's folder that is unnecessary padding/zealous for clarity, as you prefer. The difference from your example is the difference between 'the folder which went missing' and 'the folder, which went missing,' ; minute, but possibly important.