I've been wondering the same thing since I can't seem to find a use for Ascendent Materials other than exchange. I don't have any older Legendary weapons to check if they still require the materials for the final upgrade path. The Crota's End weapons/ gear still require Radiant Energy.
Ascendent Materials are still rewards for the Daily Story at levels 26 and 30. So maybe there still is a use for them? Otherwise I would expect the reward to change to Motes.
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I would also like to know this, I still have a lot of weapons and armor to upgrade with ascendent materials and cant 'ascend' to year 1 until they are fully upgraded. It was sometimes difficult to get these materials unless you played VOG all the time...maybe that is still the best source...