Damn; I thought I had a diamond spike working for me, but I can't check unfortunately
I had an AA experience solidifier right nearby, and that was definitely gathering XP, but as mine was a cow breeding farm, that could have been breeding XP rather than killing XP. You're right though - if that's an intentional change, that would be a very questionable design choice. I know that with the diamond ones you get player-kill only mob loot, which is what differentiates it from the iron ones, but I thought you got XP too. Assuming diamond spikes no longer drop XP, If I had any experience at modding, I'd be tempted to make a mini-mod that adds in some sort of emerald spike or something that will do XP and player-only loot - perhaps using a diamond spike as its base. You could even add integration so that if Actually Additions is present, it uses the emerald swords added by that