Underworld
With the return of magic came the return of Necromancy. Necromancers in the 6th Age are able to summon the ghosts of some people but not others, with the only working theory being that ghosts can either refuse, are being kept away, or some theory about how 'content' they are with being dead. Some Necromancers have mastered a spell that allows them to travel bodily to the realm of the dead and rumors are abound of secret projects to create a permanent gateway to the underworld but nothing has been made public on that front yet.
There are no Shadowlands still standing on earth. But it has been theorized that one could be created. Needless to say most governments don't appreciate the attempt.
Malfeas
Sorcerers can summon Demons and Elementals both. Early theories of them being the same fundamental beings was quashed pretty quickly. But there is still some stigma around demon summoning especially in more conservative regions of the world.
Demons will willingly tell you they are from Malfeas and have described the demon city, the infinite desert, the sea that marches against the flames, and all of the other varied hellscapes in vivid and consistent detail. The public belief is that no one has been able to actually GO to Malfeas. But curios of Malfean porcelain, green brass, and Vitriol are occasionally found on the black market so maybe some one did invent a spell or craft a gateway, who can really say?
The Wyld
If you walk to the edge of the world you keep going and get to the other side, obviously. The earth is a sphere so that is just a thing that happens. But if you wander far enough into the areas of the world that are forgotten by mankind it is possible to find Wyld pockets and the Raksha courts that live in them. Unfortunate for the Raksha the earth is very stable right now so their wyld pockets don't even stay open all the time. For now the Wyld is a curiosity. It is certainly 'real' but has been notoriously hard to research and only a very few have been able to exploit it in any reasonable way.