On my visit to the Occultist, I hit - under the tag “Salvage Dungeon Keys” - the button All Keys.
The tooltip tells you that it will delete all the nightmare and ancient dungeon keys and returns sigil dust to you, which you then can use to make new sigils.
However, doing so also deleted all the modifiers for the Spirit Brazier for Undercity. These items are not “nightmare und ancient dungeon keys” (ancient dungeon keys apparently no longer exist anyway).
Also, it is not possible to make new ones with the sigil dust that you receive. Plus - unlike dungeon keys - these things stack nicely so there is no need to recycle them.
In other words: I just lost all my undercity dungeon Spirit Brazier items.
Thank you Blizzard for the wonderful gaming experience!