Seeing the post(s) regarding Thorn's grimoire card not unlocking upon acquiring Y3 Thorn, I will paraphrase the issue. Currently, Y3 exotic weapons with a Y1 variant will only trigger unlocking the grimoire card after acquiring the Y1 version of the weapon. As I do not know the status of similar Y2 exotic weapons' grimoire unlock triggers, I will not make a broader statement than that. So, your current workaround is rolling the dice on Xûr's Legacy Exotic Heavy Weapon Engrams for the Y1 Gjallarhorn, or to acquire it through luck on the exotic weapon chest/boss loot rolls in the Y1 raids (note: said chest/boss loot rolls do not guarantee a Y1 exotic, but include the exotics in the loot table, with several exceptions not impacting Gjallarhorn, yet affecting Thorn). You should also know that the legacy engrams favor decrypting into an exotic of said type that has not been unlocked on your account (I do not believe this is an obstacle, as it should be based on your Y1 unlocks, not Y2+ unlocks). If you opt to do the Y1 raids, I believe their loot table rolls do not show such favoritism towards what you have not yet unlocked.