A thread on Reddit purports to have information on hazardless stages. One of the sources cited is the Greward3 from the post above mine, so it's probably legit. I'll bold the stages that stand out to me
Link:
https://www.reddit.com/r/smashbros/comments/9u2zio/compilation_of_information_about_hazardless/
Summary:
Pokemon Stadium: No transformations
Super Happy Tree: Still has the clouds.
Duck Hunt: No ducks/dog
Reset Bomb Forest: No transformations
Warioware: No transformations
Delfino Plaza: Still transforms
Hyrule Castle: No tornadoes
Mute City SNES: Static layout, ground still hurts you
Yoshi's Island: Middle platform doesn't tilt, no Shy Guys or Whimsical Support Ghost
Mushroom Kingdom: Weight sensitive platforms gone, no Piranha Plants
Living Room: No change (???)
Green Greens: No blocks/bomb blocks, no mention of Whispy Woods and apples
Jungle Japes: No Klaptrap, water still carries you offscreen
Unova Pokemon League: Flat + 2 platforms, Pokemon appear but do nothing
Dream Land: No wind
Boxing Ring: No change
Suzaku Castle: No change
Castle Siege: No transformations
Norfair: No lava
Figure-8 Circuit: No karts
Saffron City: No Pokemon
Town & City: No balloons, platforms do not move all the way to blast zone
Princess Peach's Castle: No Banzai Bill, no buttons for platforms, existing platforms static
Umbra Clock Tower: No platforms
Wily Castle: No platforms, no Yellow Devil
Distant Planet: Leaf platforms don't sink under player weight, no rain, no Bulborb, no pill flowers
Paper Mario: No transformations, no desk fan blowing wind
Tomodachi Life: No change
Dracula's Castle: "Static and good"
Kongo Falls: Rock still exists, stage doesn't sink under player weight, no logs getting stuck in waterfall
Bridge of Eldin: No King Bulblin or bomb
Mario Circuit: Still transforms, no Shy Guys, background is still dangerous
Moray Towers: No change
Skyloft: No transformations
Luigi's Mansion: Pillars can't be hit
Mushroom Kingdom U: No transformations
Frigate Orpheon: No fllipping
Golden Plains: No coins
Brinstar: Stage elements are indestructible, no acid
Gerudo Valley: Indestructible bridge, no Twinrova
Spirit Tracks: "Stage doesn't move at all" (relative to screen or tracks?), platforms sometimes float overhead
Shadow Moses Island: Indestructible walls
Coliseum: No platforms (
boring...)
Wii Fit Studio: Two static platforms
The Summit: Still transforms, no fish, ice is unbreakable
Brinstar Depths: No Kraid, therefore static
Rainbow Cruise: Static stage, just the ship
Halberd: Combo Cannon does not attack, otherwise no change
Fountain of Dreams: Platforms are static
Pokemon Stadium 2: No transformations
3D Land: No spikes, otherwise no change
Smashville: Platform is static, no balloons
Wuhu Island: No transformations
Port Town Aero Dive: No racers, track is still dangerous when flying around
Kalos Pokemon League: No transformations
Arena Ferox: Starts at and remains in statues transformation
Pilotwings: Red plane only, background is still dangerous
Pirate Ship: No catapult, tornado, bombs, shipwreck, King of Red Lions, tilting
Corneria: No Arwings or laser
Venom: No Arwings
Final Destination: No change lol
Yoshi's Island Melee: Flip blocks don't react to attacks
Yoshi's Story: No Randall, no Shy Guys
Peach's Castle: No bumper, sliding platform is static
Balloon Fight: No bumpers/fish/lightning, stalactites are indestructible
Mario Maker: Inconclusive, need more data re: random layouts
Lylat Cruise: No tilting
Gamer: No 5-Volt, need more data re: random layouts
Tortimer Island: No fruit or sharks, layout still random
Kongo Jungle: No barrel, moving platforms are static and roughly level with top platforms
New Donk City Hall: Acts like Prism Tower, not all stops are used each cycle
Edit with some thoughts:
Pokemon Stadium, Warioware, Yoshi's Island, Unova Pokemon League, Castle Siege, Umbra Clock Tower, Wily Castle, Dracula's Castle, Skyloft, Frigate Orpheon, Brinstar, Rainbow Cruise, Fountain of Dreams, Pokemon Stadium 2, Smashville, Wuhu Island, Kalos Pokemon League, Arena Ferox, Final Destination, Yoshi's Story, and New Donk City Hall should all be shoo-ins for legality assuming this information is correct. The only potential wrinkle is similar shape to existing stages, e.g. Pokemon Stadium 1 + 2 + Unova or Umbra + Wily + FD.
Duck Hunt, Reset Bomb Forest, Delfino Plaza, Mute City SNES, Green Greens, Jungle Japes, Norfair, Mario Circuit, Mushroom Kingdom U, Spirit Tracks, Halberd, Pirate Ship, and Tortimer Island will need
honest testing to see if they're (still?) abuseable or not. Mario Maker and Gamer too, pending more data on their random layouts.
Everything else in the list is probably still banned for one reason or another. (Unless someone wants to argue that Distant Planet's sloped walkoff is somehow fine?)
Remaining stages I'm really interested in learning about are Mario Maker, Gamer, (random layouts), Magicant (bottom campy platform), Garden of Hope, Find Mii, Windy Hill Zone, and Big Blue.