Letting people choose whether to be referred to as men, women, or neutral (perhaps using neo-pronouns or singular them/they) is perfectly fine. But re-purposing existing words with other meanings is not. And "it" has a very specific meaning, one that just doesn't apply to conscious adult people. If I asked to be called "the object" that would not be a reasonable request, anymore than asking to be referred to as "it".

And particularly for "it" and other objectifying language, it has the huge problem that it makes other people uncomfortable. Especially those who overhear without being aware of your preference (imagine sending an email to a new client with text like "in my absence, contact my colleague X, it will assist you, it is an expert").