A simple utility class for limiting rotation in Unity.

RotationUtility.cs

public static class RotationUtility
{
    /// <param name="rotation">
    ///  Value of 0f to 360f.
    ///  Rotation that will be limitted to a certain angle.
    /// </param>
    public static float LimitRotation(
        float rotation,
        float minRotation,
        float maxRotation)
    {
        if (minRotation > maxRotation)
        {
            throw new ArgumentException("Min rotation cannot exceed the max!");
        }

        if (rotation < 0f ||
            rotation > 360f)
        {
            throw new ArgumentException("Rotation has to be a value in range from 0f to 360f!");
        }

        if (rotation > maxRotation ||
            rotation < minRotation)
        {
            float rotationDistanceFromMaxValue = Mathf.Abs(Mathf.DeltaAngle(rotation, maxRotation));
            float rotationDistanceFromMinValue = Mathf.Abs(Mathf.DeltaAngle(rotation, minRotation));

            if (rotationDistanceFromMaxValue < rotationDistanceFromMinValue)
            {
                return maxRotation;
            }
            return minRotation;
        }

        return rotation;
    }
}