Is the Netscape SMIME extension still required in a certificate?

Question

Every Windows 7 host has a Trusted Root certificate from Entrust that has the "Netscape Cert Type" extension (see image)

I'm comparing inter-operable S/MIME extensions and see a pretty diverse set of offerings and configurations.

Are there clients that still refer to this extension? Is the Netscape extension needed or beneficial?

Below is the full dump of the certificate:

C:\>certutil -dump c:\netscape.cer
X509 Certificate:
Version: 3
Serial Number: 374ad243
Signature Algorithm:
    Algorithm ObjectId: 1.2.840.113549.1.1.5 sha1RSA
    Algorithm Parameters:
    05 00
Issuer:
    CN=Entrust.net Secure Server Certification Authority
    OU=(c) 1999 Entrust.net Limited
    OU=www.entrust.net/CPS incorp. by ref. (limits liab.)
    O=Entrust.net
    C=US

 NotBefore: 5/25/1999 12:09 PM
 NotAfter: 5/25/2019 12:39 PM

Subject:
    CN=Entrust.net Secure Server Certification Authority
    OU=(c) 1999 Entrust.net Limited
    OU=www.entrust.net/CPS incorp. by ref. (limits liab.)
    O=Entrust.net
    C=US

Public Key Algorithm:
    Algorithm ObjectId: 1.2.840.113549.1.1.1 RSA (RSA_SIGN)
    Algorithm Parameters:
    05 00
Public Key Length: 1024 bits
Public Key: UnusedBits = 0
    0000  30 81 87 02 81 81 00 cd  28 83 34 54 1b 89 f3 0f
    0010  af 37 91 31 ff af 31 60  c9 a8 e8 b2 10 68 ed 9f
    0020  e7 93 36 f1 0a 64 bb 47  f5 04 17 3f 23 47 4d c5
    0030  27 19 81 26 0c 54 72 0d  88 2d d9 1f 9a 12 9f bc
    0040  b3 71 d3 80 19 3f 47 66  7b 8c 35 28 d2 b9 0a df
    0050  24 da 9c d6 50 79 81 7a  5a d3 37 f7 c2 4a d8 29
    0060  92 26 64 d1 e4 98 6c 3a  00 8a f5 34 9b 65 f8 ed
    0070  e3 10 ff fd b8 49 58 dc  a0 de 82 39 6b 81 b1 16
    0080  19 61 b9 54 b6 e6 43 02  01 03
Certificate Extensions: 8
    2.16.840.1.113730.1.1: Flags = 0, Length = 4
    Netscape Cert Type
        SSL CA, SMIME CA, Signature CA (07)
    2.5.29.31: Flags = 0, Length = 110
    CRL Distribution Points
        [1]CRL Distribution Point
             Distribution Point Name:
                  Full Name:
                       Directory Address:
                            CN=CRL1
                            CN=Entrust.net Secure Server Certification Authority
                            OU=(c) 1999 Entrust.net Limited
                            OU=www.entrust.net/CPS incorp. by ref. (limits liab.)
                            O=Entrust.net
                            C=US
        [2]CRL Distribution Point
             Distribution Point Name:
                  Full Name:
                       URL=http://www.entrust.net/CRL/net1.crl

    2.5.29.16: Flags = 0, Length = 24
    Private Key Usage Period
    Unknown Extension type

    0000  30 22 80 0f 31 39 39 39  30 35 32 35 31 36 30 39   0"..199905251609
    0010  34 30 5a 81 0f 32 30 31  39 30 35 32 35 31 36 30   40Z..20190525160
    0020  39 34 30 5a                                        940Z
0000: 30 22                                     ; SEQUENCE (22 Bytes)
0002:    80 0f                                  ; CONTEXT_SPECIFIC[0] (f Bytes)
0004:    |  31 39 39 39 30 35 32 35  31 36 30 39 34 30 5a     ; 19990525160940Z
0013:    81 0f                                  ; CONTEXT_SPECIFIC[1] (f Bytes)
0015:       32 30 31 39 30 35 32 35  31 36 30 39 34 30 5a     ; 20190525160940Z

    2.5.29.15: Flags = 0, Length = 4
    Key Usage
        Certificate Signing, Off-line CRL Signing, CRL Signing (06)

    2.5.29.35: Flags = 0, Length = 18
    Authority Key Identifier
        KeyID=f0 17 62 13 55 3d b3 ff 0a 00 6b fb 50 84 97 f3 ed 62 d0 1a

    2.5.29.14: Flags = 0, Length = 16
    Subject Key Identifier
        f0 17 62 13 55 3d b3 ff 0a 00 6b fb 50 84 97 f3 ed 62 d0 1a

    2.5.29.19: Flags = 0, Length = 5
    Basic Constraints
        Subject Type=CA
        Path Length Constraint=None

    1.2.840.113533.7.65.0: Flags = 0, Length = c
    Unknown Extension type

    0000  30 0a 1b 04 56 34 2e 30  03 02 04 90               0...V4.0....
0000: 30 0a                                     ; SEQUENCE (a Bytes)
0002:    1b 04                                  ; GENERAL_STRING (4 Bytes)
0004:    |  56 34 2e 30                                       ; V4.0
0008:    03 02                                  ; BIT_STRING (2 Bytes)
000a:       04
000b:       90

Signature Algorithm:
    Algorithm ObjectId: 1.2.840.113549.1.1.5 sha1RSA
    Algorithm Parameters:
    05 00
Signature: UnusedBits=0
    0000  b2 f9 06 ed ea 93 0a f7  0b 45 2d 6d 3e 52 f9 05
    0010  5c b5 19 73 54 a0 87 78  32 ab ef 05 ad ba 1f 2c
    0020  a3 62 2f 96 22 0e 58 8c  43 60 c3 d9 1d d7 9f 3d
    0030  e2 3c f6 72 ba 32 87 a8  bb 98 d3 f4 18 4f 46 ef
    0040  11 7a 84 75 57 e3 3f ee  e7 c6 31 c8 65 36 26 ce
    0050  2c bb 04 6a 95 d5 d5 c3  f1 be 92 f7 0d 10 c5 f8
    0060  8c ef b5 98 e4 fb 0a 13  8d 7c 25 ff 0e 47 fb a8
    0070  17 34 8d 21 7c a5 0a a7  c2 74 64 fa 02 30 dc 90
Signature matches Public Key
Root Certificate: Subject matches Issuer
Key Id Hash(rfc-sha1): ea bf 10 a5 01 1e 95 8f c9 55 b4 67 60 a4 8f fd 8b 0f 12 b4
Key Id Hash(sha1): f0 17 62 13 55 3d b3 ff 0a 00 6b fb 50 84 97 f3 ed 62 d0 1a
Cert Hash(md5): df f2 80 73 cc f1 e6 61 73 fc f5 42 e9 c5 7c ee
Cert Hash(sha1): 99 a6 9b e6 1a fe 88 6b 4d 2b 82 00 7c b8 54 fc 31 7e 15 39
CertUtil: -dump command completed successfully.

C:\>

Answer 1

This extension was used during the period IE4, IE5 and Netscape were popular back in the '90s. I'm unsure if any modern implementation even looks for this extension.

Since every certificate in the Windows 7 root store with the Netscape extension also has a key length of 1024 bits, and an expiration of before 2020 I would assume that this extension is obsolete and should no longer be used in newer certificates.

In comparison all newer certificates use key lengths of 2048, or hashes like SHA2 that would likely break such a legacy implementation.

Implementing this extension may introduce subtle bugs in various applications that still have legacy code that checks for this.