There are several instances on our server (Vidar), at least, that cannot be accessed even though they exist (I think? in most cases).
1. Chaos Vortex (we cleared it some months ago, and npcs for the next phase never spawned - they exist on German server i checked on).
2. Arcanium Chamber - only Hall of Earth portal is active. Rest of the portals are disabled for several months now (was ninja-changed, before the portals and mobs to farm fragments were there). And you can't farm keys for them since mobs don't spawn now. So Hall of Water, Hall of Fire, and Hall of Chaos.
Most people will probably not care because they are not giving good gear any more but it would be nice if players could clear this content, and as it has been designed already I don't think activating it would take too much manpower (idk the process).