There had been an update for the Safari 11.1.2 ~ & also some means to upgrade
the security to later version higher levels, to access sites otherwise inaccessible.
..Safari 11.1.2 has 'Security Update' listed here..
• Apple security updates - Apple Support
https://support.apple.com/en-us/HT201222
(July 9 2018 : confirmed safari update info)
• Download Security Update 2018-004 (El Capitan)
https://support.apple.com/kb/DL1972?locale=en_US
This as close is you'd get with Safari 11.1.2 Security update:
https://updates.cdn-apple.com/2019/cert/041-90801-20191011-d72312e9-c765-400a-b3bb-889f86d2ac7d/SecUpd2018-004ElCapitan.dmg
The later issue/answer addresses Trust Certificates; each may be
found by tedious search or read through. Perhaps trial & error, in
a matter of speaking.. ~ Some listed in main Support page aren't
the ones you'd need in El Capitan. Others are; or could be..!
• invalid certificate - Apple Community
https://discussions.apple.com/thread/251429106?answerId=252787036022#252787036022
Note the list of Certificates within linked Apple support section.
Of these, 'some' will not work to upgrade various Certificates.
• Add Trust External CA Root" Certificate - ASC (links newer Apple certificate)
https://discussions.apple.com/thread/251422116?answerId=252786322022#252786322022
The old outdated Certificates need be removed from KeyChain
and put into Trash.. (read linked page for Trust Certificates)
a List of ALL the Certificates ~ including those you do not need
Go to the Apple certificate page:
https://www.apple.com/certificateauthority/
(El Capitan in my macs uses Firefox latest version; said to maybe
not be updated in the future, for Mac; hopeful it'd stick around)
Take care & good luck!☀️🌙