That’s what I had assumed until I saw two separate people claim you could get a recipe for it from The Judge. Memory can be fickle though, they could be mistaken.
It’s also possible that for a brief moment in time it did drop. Things do get changed especially if they were unintentional.
When mobs began dropping the armor they were wearing it was possible to get rusted armor from surge NPCs and dismantle it for the armor shards required to make champion and godbreaker armor. That got changed real fast even though afaik it’s not possible to get those recipes on Siptah either.
I can say I’ve never seen it drop and that the last time I checked there is no recipe scroll for heavy silent legion in the admin panel. Though it’s been a while.
If you happen to be on 6436 or 6442 I can make you a set via transferred characters if you like.
If you’re a real lover of heavy armor, then all the above are nothing in front the vault armor pieces. Eldarium recipes are the strongest on the game.
But yes, the dragon bone is extremely cheap armor for what it gives .