CustomItemSerializer.cs

public interface ICustomItemSerializer : IItemSerializer
{
}

public class CustomItemSerializer : DefaultItemSerializer, ICustomItemSerializer
{
    /// <summary>
    /// TemplateId - FieldIds to exclude
    /// </summary>
    private static readonly IDictionary<ID, ID[]> _fieldsToExclude = new Dictionary<ID, ID[]>()
    {
        {
            new ID(Constants.Template1.TemplateId),
            new ID[]
            {
                Constants.Template1.Fields.Field1,
                Constants.Template1.Fields.Field2,
            }
        }
    };

    public CustomItemSerializer(IGetFieldSerializerPipeline getFieldSerializerPipeline) : base(getFieldSerializerPipeline)
    {
    }

    protected override IEnumerable<Field> GetItemFields(Item item)
    {
        if (!_fieldsToExclude.TryGetValue(item.TemplateID, out var fields))
        {
            return base.GetItemFields(item);
        }

        return base.GetItemFields(item).Where(f => !fields.Contains(f.ID));
    }
}