[danabramov]

I think most people's mental model is that they should be able to change their handle / display name / avatar freely, and their posts would display the new versions. So those aren't a part of the post itself.

That said, you could create an AT app that displays a version of the post using the profile at the time. You'd just need to index all profile changes into a local database, and then query that database for the "profile at that time" to display the post. So what you're describing is possible—it just requires a different aggregation. The source of truth, however, should be denormalized and reflect most recent data.