tl;dr: Change to calling map { ... } on the LiveData object, change the variable to its nullable version.

It seems like you are on the right track with how you tried to fix this. With v2.6.0 of the Lifecycle library, the Transformations was completely removed (which you unfortunately had to find out the hard way), but thankfully the code is simpler now.

For a concrete example, this code:

this.currentPlayer = Transformations.map(this.currentGame) { gameWithPlayers ->
    gameWithPlayers?.players?.firstOrNull { it.isRolling }
}

is now

this.currentPlayer = this.currentGame.map { gameWithPlayers ->
    gameWithPlayers?.players?.firstOrNull { it.isRolling }
}

But, as you discovered, this causes issues with nullability due to how newer versions of LiveData handle null values. Luckily, this is an easy enough fix; change the LiveData variable to have a nullable type inside of it.
Specifically, this means:

val currentPlayer: LiveData<Player>

becomes

val currentPlayer: LiveData<Player?>

That should resolve the issues you have, let you move forward, and get rid of the Transformations class which was never my favorite in the first place!