It is not disabled, you just can't currently get the perk without mods/developers console. The last time that I personally saw this perk being available was back in 1.2.0 (I skipped directly to 1.4.0 so maybe it was still available before then).
I put it in quotes because you would need to have the 275 smithing perk that lets you create legendary weapons (but you currently cannot unlock that perk due to a bug - technically you're supposed to be able to pick between the Siege perk and the Legendary smithing perk, but only one shows for some reason and currently it is the Siege perk). there's some misinformation posted above.