Found out today that only a seasonal character can supposedly access this dungeon. This is rather lame, and somewhat a let down.
Looks like the Season 10 update fixed the issue of the Stockades dungeon from being inaccessible.
Not for me. Right now standing in front of Stockades gate, no symbol on the map. But interactive map says I’m right.
So no way through gate, no symbol.
In my case it could be the missing campaign progress according to Google AI search