Get the "top" block at a given position using world.raycast.

TopCommand.java

public class TopCommand implements Command<ServerCommandSource> {
    @Override
    public int run(CommandContext<ServerCommandSource> context) throws CommandSyntaxException {
        ServerCommandSource source = context.getSource();
        ServerPlayerEntity player = source.getPlayer();
        World world = source.getWorld();
        Vec3d playerPos = player.getPos();

        int new_y;
        double new_x = playerPos.x;
        double new_z = playerPos.z;

        BlockHitResult blockHitResult = world.raycast(new RaycastContext(
            new Vec3d(new_x, world.getHeight(), new_z),
            new Vec3d(new_x, 1, new_z),
            RaycastContext.ShapeType.COLLIDER,
            RaycastContext.FluidHandling.SOURCE_ONLY,
            player
        ));
        new_y = blockHitResult.getBlockPos().getY() + 1;
      
        return new_y;
    }
}