Making blocks directional (facing into certain directions) is also done using block states. This example describes a vertical version of the andesite slab.

VerticalSlabBlock.java

public class VerticalSlabBlock extends HorizontalFacingBlock {
	// the codec is required since 1.20.5 however not actually used in Minecraft yet.
	public static final MapCodec<VerticalSlabBlock> CODEC = Block.createCodec(VerticalSlabBlock::new);
 
	public VerticalSlabBlock(Settings settings) {
		super(settings);
		setDefaultState(getDefaultState().with(Properties.HORIZONTAL_FACING, Direction.NORTH));
	}
 
	@Override
	protected MapCodec<? extends VerticalSlabBlock> getCodec() {
		return CODEC;
	}
 
	@Override
	protected void appendProperties(StateManager.Builder<Block, BlockState> builder) {
		builder.add(Properties.HORIZONTAL_FACING);
	}
 
	@Override
	public VoxelShape getOutlineShape(BlockState state, BlockView world, BlockPos pos, ShapeContext ctx) {
		Direction dir = state.get(FACING);
		return switch (dir) {
			case NORTH -> VoxelShapes.cuboid(0.0f, 0.0f, 0.0f, 1.0f, 1.0f, 0.5f);
			case SOUTH -> VoxelShapes.cuboid(0.0f, 0.0f, 0.5f, 1.0f, 1.0f, 1.0f);
			case EAST -> VoxelShapes.cuboid(0.5f, 0.0f, 0.0f, 1.0f, 1.0f, 1.0f);
			case WEST -> VoxelShapes.cuboid(0.0f, 0.0f, 0.0f, 0.5f, 1.0f, 1.0f);
			default -> VoxelShapes.fullCube();
		};
	}
 
        @Override
	public BlockState getPlacementState(ItemPlacementContext ctx) {
		return super.getPlacementState(ctx).with(Properties.HORIZONTAL_FACING, ctx.getHorizontalPlayerFacing().getOpposite());
	}
 
}

and then register the block according to the method covered in blocks:

TutorialBlocks.java

public final class TutorialBlocks implements ModInitializer {
    [...]
 
    public static final VerticalSlabBlock POLISHED_ANDESITE_VERTICAL_SLAB = register("polished_andesite_vertical_slab", 
        new VerticalSlabBlock(Block.Settings.copy(Blocks.POLISHED_ANDESITE)));
}

src/main/resources/assets/tutorial/blockstates/polished_andesite_vertical_slab.json

{
  "variants": {
    "facing=north": { "model": "tutorial:block/polished_andesite_vertical_slab", "uvlock": true },
    "facing=east":  { "model": "tutorial:block/polished_andesite_vertical_slab", "y":  90, "uvlock": true },
    "facing=south": { "model": "tutorial:block/polished_andesite_vertical_slab", "y": 180, "uvlock": true },
    "facing=west":  { "model": "tutorial:block/polished_andesite_vertical_slab", "y": 270, "uvlock": true }
  }
}

src/main/resources/assets/tutorial/models/block/vertical_slab.json

{   "parent": "block/block",
    "textures": {
        "particle": "#side"
    },
    "elements": [
        {   "from": [ 0, 0, 0 ],
            "to": [  16, 16, 8 ],
            "faces": {
                "down":  { "texture": "#bottom", "cullface": "down" },
                "up":    { "texture": "#top",    "cullface": "up" },
                "north": { "texture": "#side",   "cullface": "north" },
                "south": { "texture": "#side",   "cullface": "south" },
                "west":  { "texture": "#side",   "cullface": "west" },
                "east":  { "texture": "#side",   "cullface": "east" }
            }
        }
    ]
}

src/main/resources/assets/tutorial/models/block/polished_andesite_vertical_slab.json

{
    "parent": "tutorial:block/vertical_slab",
    "textures": {
        "bottom": "block/polished_andesite",
        "top": "block/polished_andesite",
        "side": "block/polished_andesite"
    }
}

For directional blocks, you may have to override rotate and mirror methods, so that in structure blocks, they can be correctly rotated or mirrored. However, in this case, the HorizontalFacingBlock class has already done it for you.

If you place these blocks in game, you may find the issue that, the mobs when trying to find paths, will try to cross the blocks, as if the blocks did not exist, ending up being blocked by the blocks. That's because mobs treat such non-full-cube blocks as those not blocking their path. To modify this path-finding behavior, you need to override canPathfindThrough method:

  @Override
  protected boolean canPathfindThrough(BlockState state, NavigationType type) {
    return false;
  }

In vanilla, some blocks have different path node types. For example, mobs will avoid some dangerous blocks, such as magma block, wither rose and cactus. You can also make mobs treat your blocks differently, by modifying path node types, via LandPathNodeTypesRegistry of Fabric API.

Try to make it waterloggable.