Thanks for this, pretty interesting analysis.
Every time I come across an old post in the EA forum I wonder if the karma score is low because people did not get any value from it or if people really liked it and it only got a lower score because fewer people were around to upvote it at that time.
The other thing going on here is that the karma system got an overhaul when forum 2.0 launched in late 2018, giving some users 2x voting power and also introducing strong upvotes. Before that, one vote was one karma. I don’t remember exactly when the new system came in, but I’d guess this is the cause of the sharp rise on your graph around December 2018. AFAIK, old votes were never re-weighted, which is why if you go back through comments on old posts you’ll see a lot of things with e.g. +13 karma and 13 total votes, a pattern I don’t recall ever seeing since.
Partly as a result, most of the karma old posts have will have been from people going back and upvoting them later once the new system was impemented, e.g. from memory my post from your list was around +10 for most of its life, and has drifted to its current +59 over the past couple of years.
This jumps out to me because I’m pretty sure that post was not a particularly high-engagement post even at the time it was written, but it’s the second-highest 2015 post on your list. I think this is because it’s been linked back to a fair amount and so can partially benefit from the karma inflation.
(None of which is meant to take away from the work you’ve done here, just providing some possibly-helpful context.)