What you're seeing is a lint warning, try suppressing the lint as follows:

AnimatedContent(
    targetState = uiState.singleId,
    transitionSpec = {
        if (targetState > initialState)
            slideInHorizontally { it } + fadeIn() togetherWith slideOutHorizontally { -it }
        else
            slideInHorizontally { -it } + fadeIn() togetherWith slideOutHorizontally { it }
    },
    label = "Fade Content",
) { @Suppress("UnusedTransitionTargetStateParameter") _ ->
    Content(dat = dat, uiState = uiState, onUiEvent = onUiEvent)
}