LI experience as a quest reward – not IXP runes – still works, the item experience gets converted to character experience. My Level 75 character just did three grey Level 55 repeatable quests in The Weather Hills (east of Bree). Each quest displayed LI experience as a reward. Upon completion, my character got a few thousand experience points for each quest – grey quests would include only a few hundred experience each so the extra must be from a conversion of the LI experience reward.
We've been told that IXP runes and other items which applied to the old LI system will eventually be tradeable for something which applies to the new LI system.