What happens if I have expired additional certificate in the chain with alternate trust path?

That day has come. On 30th May 2020 the USERTrust RSA Certification Authority has expired and I see that modern browsers works correctly, but most automatics isn't (e.g. curl from Ubuntu 18.04 LTS console and from PHP and Ruby scripts).

One of SSL distributers said:

Previously they [Sectigo] assured everyone that no issues will be. However, the reality is that some legacy servers/devices are affected.

That is a ridiculous situation. We pointed their attention to the expiring AddTrust RSA/ECC multiple times within a year and each time Sectigo assured us no issues will be.

Sectigo issued faq page.

The right chains in my case (Sectigo DV product / Positive SSL):

For RSA:

# Subject: /C=GB/ST=Greater Manchester/L=Salford/O=Sectigo Limited/CN=Sectigo RSA Domain Validation Secure Server CA
# Algo: RSA, key size: 2048
# Issuer: /C=US/ST=New Jersey/L=Jersey City/O=The USERTRUST Network/CN=USERTrust RSA Certification Authority
# Not valid before: 2018-11-02T00:00:00Z
# Not valid after: 2030-12-31T23:59:59Z
# SHA-1 Fingerprint: 33:E4:E8:08:07:20:4C:2B:61:82:A3:A1:4B:59:1A:CD:25:B5:F0:DB
# SHA-256 Fingerprint: 7F:A4:FF:68:EC:04:A9:9D:75:28:D5:08:5F:94:90:7F:4D:1D:D1:C5:38:1B:AC:DC:83:2E:D5:C9:60:21:46:76
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

# Subject: /C=US/ST=New Jersey/L=Jersey City/O=The USERTRUST Network/CN=USERTrust RSA Certification Authority
# Algo: RSA, key size: 4096
# Issuer: /C=GB/ST=Greater Manchester/L=Salford/O=Comodo CA Limited/CN=AAA Certificate Services
# Not valid before: 2019-03-12T00:00:00Z
# Not valid after: 2028-12-31T23:59:59Z
# SHA-1 Fingerprint: D8:9E:3B:D4:3D:5D:90:9B:47:A1:89:77:AA:9D:5C:E3:6C:EE:18:4C
# SHA-256 Fingerprint: 68:B9:C7:61:21:9A:5B:1F:01:31:78:44:74:66:5D:B6:1B:BD:B1:09:E0:0F:05:CA:9F:74:24:4E:E5:F5:F5:2B
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

For ECC:

# Subject: /C=GB/ST=Greater Manchester/L=Salford/O=Sectigo Limited/CN=Sectigo ECC Domain Validation Secure Server CA
# Algo: EC secp256r1, key size: 256
# Issuer: /C=US/ST=New Jersey/L=Jersey City/O=The USERTRUST Network/CN=USERTrust ECC Certification Authority
# Not valid before: 2018-11-02T00:00:00Z
# Not valid after: 2030-12-31T23:59:59Z
# SHA-1 Fingerprint: E8:49:90:CB:9B:F8:E3:AB:0B:CA:E8:A6:49:CB:30:FE:4D:C4:D7:67
# SHA-256 Fingerprint: 61:E9:73:75:E9:F6:DA:98:2F:F5:C1:9E:2F:94:E6:6C:4E:35:B6:83:7C:E3:B9:14:D2:24:5C:7F:5F:65:82:5F
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

# Subject: /C=US/ST=New Jersey/L=Jersey City/O=The USERTRUST Network/CN=USERTrust ECC Certification Authority
# Algo: EC secp384r1, key size: 384
# Issuer: /C=GB/ST=Greater Manchester/L=Salford/O=Comodo CA Limited/CN=AAA Certificate Services
# Not valid before: 2019-03-12T00:00:00Z
# Not valid after: 2028-12-31T23:59:59Z
# SHA-1 Fingerprint: CA:77:88:C3:2D:A1:E4:B7:86:3A:4F:B5:7D:00:B5:5D:DA:CB:C7:F9
# SHA-256 Fingerprint: A6:CF:64:DB:B4:C8:D5:FD:19:CE:48:89:60:68:DB:03:B5:33:A8:D1:33:6C:62:56:A8:7D:00:CB:B3:DE:F3:EA
-----BEGIN CERTIFICATE-----
MIID0zCCArugAwIBAgIQVmcdBOpPmUxvEIFHWdJ1lDANBgkqhkiG9w0BAQwFADB7
MQswCQYDVQQGEwJHQjEbMBkGA1UECAwSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYD
VQQHDAdTYWxmb3JkMRowGAYDVQQKDBFDb21vZG8gQ0EgTGltaXRlZDEhMB8GA1UE
AwwYQUFBIENlcnRpZmljYXRlIFNlcnZpY2VzMB4XDTE5MDMxMjAwMDAwMFoXDTI4
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q8bVttHmc3Gu3ZzWDGH926CJA7gFFOxXzu5dP+Ihs8731Ip54KODfi2X0GHE8Znc
JZFjq38wo7Rw4sehM5zzvy5cU7Ffs30yf4o043l5o4HyMIHvMB8GA1UdIwQYMBaA
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MAYGBFUdIAAwQwYDVR0fBDwwOjA4oDagNIYyaHR0cDovL2NybC5jb21vZG9jYS5j
b20vQUFBQ2VydGlmaWNhdGVTZXJ2aWNlcy5jcmwwNAYIKwYBBQUHAQEEKDAmMCQG
CCsGAQUFBzABhhhodHRwOi8vb2NzcC5jb21vZG9jYS5jb20wDQYJKoZIhvcNAQEM
BQADggEBABns652JLCALBIAdGN5CmXKZFjK9Dpx1WywV4ilAbe7/ctvbq5AfjJXy
ij0IckKJUAfiORVsAYfZFhr1wHUrxeZWEQff2Ji8fJ8ZOd+LygBkc7xGEJuTI42+
FsMuCIKchjN0djsoTI0DQoWz4rIjQtUfenVqGtF8qmchxDM6OW1TyaLtYiKou+JV
bJlsQ2uRl9EMC5MCHdK8aXdJ5htN978UeAOwproLtOGFfy/cQjutdAFI3tZs4RmY
CV4Ks2dH/hzg1cEo70qLRDEmBDeNiXQ2Lu+lIg+DdEmSx/cQwgwp+7e9un/jX9Wf
8qn0dNW44bOwgeThpWOjzOoEeJBuv/c=
-----END CERTIFICATE-----

The USERTrust RSA Certification Authority in both cases are the same CA (same Subject, same key) and has been signed twice - once by the AddTrust External CA Root and once by itself.

This allows Sectigo to introduce the USERTrust RSA Certification Authority as a new CA while they work with all relying party application vendors (browsers and OSes generally) to ensure it is added to their trust anchor store. Once they are confident that it is trusted by all, the AddTrust External CA Root is redundant and can be decommissioned, which seems to be planned for May 2020.

As you expect that all browsers trust the USERTrust RSA Certification Authority certificate, there is no reason to send the longer chain. However, if you believe some of your user base is using obsolete browsers or OSes then you may need to send both. Only you can decide.

If you do decide to send both chains, relying parties will work out the chain to the trust anchor themselves and it make no difference which path they take - as long as they get to one of the Roots. Once we're into June, only one path will be valid, therefore the other will be discarded. At this point, any of your use base using obsolete browsers or OSes will get error messages regardless of whether your sending the old chain or not, so you might as well stop sending it and save Internet traffic (marginally).