"AddTrust External CA Root" Certificate Expired: Websites Won't Open

There is Mojave 10.14.6, or slightly older High Sierra 10.13.6. These do not require your system apps

or drivers to be fully 64-bit compliant; and can still use 32-bit software so that you may not need

to also get newer printer, or peripherals for 64-bit only. ~ Catalina macOS 10.15.x is solely 64-bit.

• How to upgrade to macOS Mojave 10.14.6 - Apple Support

https://support.apple.com/en-us/HT210190

Be sure to have at least two backups of your present system prior to upgrade attempts; Time

Machine can be useful to restore from, should you wish keep older stuff into a newer system;

although some users prefer all-new content.

Other options could include using third-party Browser (Firefox) and Email client (Thunderbird)

to stay with the older El Capitan 10.11.6. ~ At least until you've figured this further along.

Good luck & happy trails!🌻🌤

Another thing to try at least for Safari...

Go on the Apple certificate page:

https://www.apple.com/certificateauthority/

Download the Apple Intermediate Certificates

Apple IST CA 2 - G1 Certificate (direct download)

Double click the downloaded certificate to install it in Keychain Access.

You should see it now, in login certificates.

I had no luck with removing certificates or the endless other things I tried. But:

Solved! Known problem in El Cap, simple fix. Grrr.

Called Apple four days ago and got an incompetent tech. She essentially ended up shrugging her shoulders, no clue, sent me packing.

That started three days of Catalina upgrade and endless reboots and calls to other software firms to TRY and solve all the ensuing problems. (Lots of old apps croak, plus problems with Apple apps/services.)

Called again today and got savvy tech who's been using Macs since '86 (newbie, by my lights... ;-) ).

Just upgrade El Cap to Sierra. Sweet mother Mary. If that first tech had just told me that...

Sierra: https://support.apple.com/kb/HT208202

The install program on this DMG installs an installer in your Applications folder. Run that.

Other, later installers:

High Sierra: https://support.apple.com/kb/HT208969

Mojave: https://support.apple.com/kb/HT210190

According to actual experience (and wikipedia) Safari version 11.1.2

for El Capitan 10.11.6, support faded (July 9, 2018). While I sought info

about Opera, lacked detail for their recent version or present status.

The "Brave" web browser is supported/runs in El Capitan 10.11.6; and

may be OK once details using it, are understood. https://brave.com/

The Opera background changed with their adapting Chrome features;

some macOS users at that time, had issue with Opera. I'd not used it

since just before major transitions; & it didn't work for me or my mac.

There had been some ability to change User Agents in certain browsers

to see if they still be useful after their sunset; some did OK for awhile. I'd

not tried this recently. (With older vintage Mac OS X, I'd tried to do that.)

The certificates idea -in previous situations- was workable; there may be

path through this shortcoming by using the 'Apple IST CA 2 - G1 Cert.'

{..Thanks BDAqua for this; to see if that's an option.!}

Good luck & happy trails!🌤☘

Expired ones are doing any good so you could delete them.

I like K Shaffer's idea of Brave browser. :)

Hello,

macOS 10.11 El Capitan is becoming increasingly more incompatible with websites. Your Macintosh supports the latest release of macOS.

How to upgrade to macOS 10.15 Catalina - Apple Support

Great news, take care my friend. :)

Thanks for the report! :)