[3º Improvement]

Lead.cs

public class Lead
{
    public Lead(string name, string email)
    {
        Name = name;
        Email = email;
        _segments = new List<Segment>();
    }

    public string Name { get; }

    public string Email { get; }

    private readonly List<Segment> _segments;
    public IReadOnlyList<Segment> Segments => _segments.ToList();

    public string PhoneNumber { get; private set; }

    public string Address { get; private set; }

    public bool? Gender { get; private set; }

    public DateTime? BirthDate { get; private set; }

    public void CompleteInfo(string phoneNumber, string address, bool gender, DateTime birthDate)
    {
        PhoneNumber = phoneNumber;
        Address = address;
        Gender = gender;
        BirthDate = birthDate;
    }

    public void AddSegment(Segment segment)
    {
        _segments.Add(segment);
    }
}