It/them

[Xinfu]

Could I use it?(not them)

-Honorifics were important to people as __ showed one thing: whether the speaker showed respect.

My friend says the antecedent is honorifics, so them is the only possible form, but my opinion is different: it represents the non-expressed act of using honorifics, which act is singular, justifying the use of it, not them. Do you agree with me?

[Rusty]

'Them' is incorrect. You need a subject pronoun.
'They' is the correct subject pronoun, since the antecedent is plural.

'It' could be used if the author switched gears mid-sentence and began thinking about the 'use/usage of honorifics' instead.

(The subject pronoun 'it' happens to share the same word as the object pronouns, but the subject pronoun 'they' does not share the same word as the object pronouns.)

Switching can happen when a generalized subject (appearing in the plural) becomes more personal/particular to the author as the thought progresses.

[Xinfu]

Excellent answer. Thank you.