Constructions

Q. Can I use Split card?

A. According to 708.4b, That mana value is sum of those. You can use that card if the mana value is 5 or more. For example, Failure // Comply is illegal, but Rags // Riches is legal.

Q. Can I use Adventurer card?

A. According to 715.4, An adventurer card is a creature card in every zone except the stack. You can use that card if the mana value is 5 or more.
For example, Oakhame Ranger is illegal, but Beanstalk Giant is legal.

Q. Can I use Companion?

A. Same as original Commander format, You can specifying the card as Companion and as 51th card of your deck if the Color Identity of that is match or subset of your Commander. Naturally, required the mana value is 5 or more.

Note

But, Keruga, the Macrosage was banned with this format in 2020/12/07.

Q. In case of Modal Double-Faced Card that Front face is not land but Back face is land, Can I use that?

A. According to 711.4a, Modal Double-Faced card in Outside the Game is only have the Characteristics of front face. You can use that if the mana value of front face is 5 or more, or front face is land. For example, Tangled Florahedron is illegal.

Info

Modal Double-Faced Cards that mana value of front face is 5 or more and back face is not is printed in Kaldheim(Alrund, God of the Cosmos and Tergrid, God of Fright). Certeinly you can use those.