These are the dimensions my players have had direct contact with. They're getting high level and this stuff is coming up more and more frequently, sometimes in worrying ways. They haven't traveled personally to any other dimensions yet (pretty sure), but have dealt with interlopers from or have had to deal with the effects of them.
Reality -- this of course is where everything happens
Plane of Fire -- they've met a handful of conjured fire elementals, and found a puzzlebox that seems connected to the plane of fire somehow. Most recently they've met the elemental Salliflax, who keeps the Flying Fortress of the Sky Witch afloat. He's basically Calcifer.
Hell -- they helped a young noblewoman at a black market auction buy a magical diving bell which can transport those inside directly to Hell. What is Hell? Common knowledge says devils come from Hell, and bad people go to Hell when they die, but the slain Demon Lords were on Earth from the start. Players have not yet made the journey to Hell. It's hard to find the time, but the noblewoman who took the trip reported that she had a direct view of Castle Satan.
Realm of Fenrath, God of Wolves -- in very first adventure, players found a magic glass that looked into a vast, darkened forest. The gods retreated to their separate realms after they defeated the demon lords, and do not venture out.
"Hot Car Dimension" -- players looted a ring from a trio of beast-headed demons that can conjure a scalding hot door, behind scalding hot door is a small, metal, scalding hot room. They put things in there they want to forget about, like frost demons. They aren't sure of its true nature but it's definitely another plane or demiplane. Hot Car Dimension is just what they call it.
Whatever's embedded in the Hyperplasmic Gap of the Silk Wall -- they found a path through the massive wall that separates the Blighted Lands from Orostranthy, but when they looked within, they found a great ocean of gold, a foreign sky, and an alien creature sunning itself on obsidian plinths. They declined to explore and flew over the wall instead.
Whatever's surrounding the Thrice Cursed Prison of Dendric Vast -- crawling over the ramparts from the inside of the prison leads to a darkened, blighted landscape covered in ruins and alien creatures. Three alien moons float in the sky, the stars are too close and bright. What is this place?
Other possible planes of existence?
Other Elemental Planes -- I'm not sure they've ever met or conjured other elementals than fire, but given the spell options they must exist, right?
Plane of Slime -- they found a triumvirate of slaad conjuring a river of slime from a tortured unicorn. Also issuing forth, a variety of horrible insect monsters. Killing the slaad closed the portal, so no chance to explore further.
Carcosa -- they've stumbled on a handful of references to this place. Might be an alien planet, might be another plane of existence, maybe it's somewhere in reality?
Other "stars" and "planets" -- these seem to be places one could travel to. They found a magic orrery that lets them project their mind to these places. They found hideous monsters of alien intelligence that blasted them with psychic assaults, among other mysteries.
The Other Celestial Realms -- where are they? how does one get there? How many exist? It's widely known Belm, Izorides, and Otin have retreated to their own realms, but are there others? The wizard Malagon of the Cruelest Eye desires to kill a god and steal their blood for his divine crucible, in order to forge a ring of flight, but cannot find a way in.
Astral Plane -- Astral plane, what's that? It's on the 9th level spell list so it must exist, but . . .
Wherever demons come from that's not reality and not Hell -- they've met many many demons that are summoned or at least seem to appear from somewhere else, and that place is not Hell, probably. So.. what is it?
Planes that do not exist
Heaven -- absolutely not
Ethereal Plane -- I disallowed the spell etherealness because I don't like how it lets you phase through objects so freely, and because I think the ethereal plane is lame and boring.
Any of the other standard D&D outer planes, like, I don't know, Limbo, and any alignment focused planes. They are so many of them I can't keep them straight and I don't like the monsters.
Shadowfell and Feywild -- just not my jam
The Abyss -- Cool high fantasy concept but I prefer demons to be a little more malleable and variable. The word "demon" seems to be a catch-all for a variety of horrible spirits and monsters, most of them connected to or allied with the slain Demon Lords, but many of them not. Anyway, the Abyss as imagined in second through fifth edition D&D does not exist.
I think that covers it for now.