I get that. Irrespective of if Shakespeare used they/them in this way or not; I would challenge anyone to even have a conversation about someone, or tell a story and not use they/them to describe a singular individual. Fact is, it's so common in speech, that people breeze past it without thinking about it, because it's natural to use they/them as the indirect singular. Everyone has used they/them this way, and continue to use those words this way, without even realizing they're doing it.... like I just did.

The discussion of when/where/who started it, isn't really material to the point that almost everyone uses it in this manner right now, even if they're not introspective or analytical enough to realize they're doing it.... so I would argue that it's not relevant to focus on the who/what/where/when/why of the use of the words in this context, but rather focus on what's happening now.

Leave the discussions of who/what/where/when/why/how to the historians and the linguists. I agree that such discussions just muddy the waters of the reality of what the common usage of the words are, and therefore it should be set aside and left alone; at least by anyone who doesn't hold a PhD in English literary studies or something...