Fowler’s “that/which” distinction and the establishment of a linguistic rule

In "A Dictionary of Modern English Usage" (1926), H. W. Fowler proposed a distinction between relative pronouns: "If writers would agree to regard 'that' as the defining relative pronoun, & 'which' as the non-defining, there would be much gain both in lucidity & in ease." As Ben Yagoda observes in a recent