I def like this notion, but would really only want it for posts and top level comments.

I would greatly appreciate if this were implemented modularly (that is: in such a way that we could toggle the kinds of content that are filtered based on account age, so as to allow me to ignore posts and top level comments, but so other could also disable each of those as they see fit).

Seconded

There’s the New Account Google Highlightenator under General settings, isn’t that enough?

This!