The Edict of Nikea forced the legions to ban the biggest threat to the forces of chaos and ensured the thousand sons eventual turn, as, in a typically stupid short sighted move by the emperor he banned the use of Psychic powers other then Astropaths and Navigators.
I know that during the time of the Heresy the edict was instigated to different levels by the loyal forces. Guilliman states at Calth that his first act after the war will be to ensure his father lifts the ban, the Darkangels also lifted the ban during the Heresy, Corax flip flopped, the space wolves don't have psykers (just like they don't have wolves on Fenris) but the Imperial Fists, in typical fashion locked theirs up and decided they could meditate the psyker away.
But when was the ban officially lifted, when where the legions and later chapters allowed to use Psykers again without fear of reprisal? Did the Emperor lift it prior to the Seige of terra, or was it done post golden throne internment? Or was it never truly ended, everyone just ignored it understanding that Psykers where a necessary evil in the fight against chaos?