it wouldn’t even be that hard to implement a historical series of diffs

And external services provide this as well, like those services where you can find deleted comments (or the internet archive).

I just try to disassociate my identity as much as I can from sites like Reddit. I never used my email on Reddit, and I haven't used mine here. I'm guessing an enterprising individual could triangulate who I am based on my posts (though I do post false information sometimes), but that's a lot less likely than if I handed over that association (i.e. through Facebook or whatever).

Do what you can, but yeah, assume that everything you post on the internet exists forever.