How did "that" get substituted for "who" or "whom" in common usage? [closed]

There is a fairly widespread analysis according to which "that" in restrictive relative clauses is not a relative pronoun, but is rather a clause introducer. See, for instance McCawley The Syntactic Phenomena of English. In this case, use of "that" rather than a wh-pronoun has nothing to do with somehow objectifying people.

Elsewhere, "that" introduces subordinate clauses -- in noun complements, such as "the fact that he disappeared", and in verb complements, such as "know that he disappeared". And unlike a real relative pronoun, "that" in a relative clause always comes at the beginning of the clause -- compare "the book about which he talked" with *"the book about that he talked".

That as a pronoun is an old usage that dates back to Old English:

• Old English þæt, "that, so that, after that," neuter singular demonstrative pronoun ("A Man's a Man for a' that"), relative pronoun ("O thou that hearest prayer").....

• From Proto-Germanic *that, from PIE *tod-, extended form of demonstrative pronominal base -to-. With the breakdown of the grammatical gender system, it came to be used in Middle English and Modern English for all genders.

Etymonline