In Gen 8, Clefable can learn Teleport only when obtained from:
Gen 7 virtual console transfer from gen 1-2
Gen 8 Pokémon Home transfer from LGPE
> In Gen 8, Clefable can learn Soft-Boiled only when obtained from:
Gen 3 or earlier (move is level-up/tutor/TM/HM in Gen 3)
In Gens 1-2, Clefable could learn Teleport via TM. Virtual Console transfer allows you to transfer Pokemon from Gens 1-2 immediately to Gen 7, as shown here, since you can have Virtual Console for the Nintendo 3DS:
These releases are also compatible with the Poké Transporter, allowing one-way transfer of Pokémon in the player's first Box to Pokémon Bank, where the Pokémon can then be withdrawn by Pokémon Sun and Moon, Pokémon Ultra Sun and Ultra Moon or sent to Pokémon HOME. From Pokémon HOME, they can be withdrawn in a Generation VIII game.
This means we skip Gen 3, the Generation Clefable can learn Softboiled. You can get Clefable to know Softboiled via Move Tutor in the games FireRed / LeafGreen , or Emerald , if you traded with someone who has FireRed/LeafGreen to get a Clefable in Emerald.
Also, the other way Clefable can get Teleport is from LGPE, which is in Gen 7, after Gen 3. Since you can't transfer Pokemon from say, Gen 7 to Gen 3, it's impossible to get the combination of Teleport and Softboiled on Clefable.
Sources:
using the /learn command to see how Clefable learns Teleport and Softboiled on Pokémon Showdown
Virtual Console
Hope this helps.
