Soul Receptor (released in early EP3) will be an extremely important piece for making the upcoming end-game unit affix possible and we still do not have it. We currently only have weapons and they are obtainable from the CM miles shop. The easiest fix for this would be to just add units that are pre-affixed with Soul Receptor into the shop and call it a day. This would also revive CM and make it relevant again, so it's a win-win IMO.
Anyone concerned about the exclusion of SR units should ask about it where ever possible (tweets, twitch chat, player survey, etc). I fear that it might have been forgotten completely.