In a previous post I suggested it might be helpful to distinguish between grammar and style. Grammar, to recapitulate, is concerned with the ways in which a dialect allows words and sentences to be constructed. It provides us with rules like that which tells us that the past tense of regular verbs is formed by the addition of‘-(e)d’ to the plain form: walked from walk. Grammarians may disagree on various points, but, on the whole, the grammar of any particular dialect is non-negotiable.
Everything else concerns style. Style considers the type of language that might or might not be suitable for use in a particular place at a particular time for a particular purpose. About that there can a wide range of opinions. Long may the discussion of such views continue, so long as it is clear that any such discussion isn’t about grammar.
Relative clauses might serve as an example of what I mean. Integrated relative clauses in which the antecedent is an inanimate object can be introduced by which, that or by nothing. Thus, we can say:
That’s the tree which the storm blew down.
That’s the tree that the storm blew down.
That’s the tree the storm blew down.
What we can’t say is
*That’s the tree who(m) the storm blew down.
It’s ungrammatical. No one would say it. That’s a matter of fact.
Now, there is some dispute, particularly in the United States, over whether an integrated relative clause can be introduced by which rather than that. There is plenty of evidence to show that it can be. For example, Franklin D Roosevelt spoke of ‘a date which will live in infamy’. The King James Bible uses both which and that in a single sentence: ‘Render therefore unto Caesar the things which are Caesar’s; and unto God the things that are God’s.’ Whether or not a writer introduces an integrated relative clause with which is up to the writer. Those who don’t like it don’t have to use it, but what they cannot do is say that it is ungrammatical. Grammar describes features of a dialect which no one of us individually can change. What we do within its constraints is a matter of choice and debate.
 ‘Integrated’ is the term used by the authors of ‘The Cambridge Grammar of the English Language’. Others use ‘defining’ or ‘restrictive’.