Web browsing keeps saying connection not private for all websites

This can happen on older macOS versions, have you tried firefox? That browser still has extended support for ElCap for as far as I know

Also, a 2015 MBA can update to Monterey. You might want to give that a try to make sure this issues will go away

..meaghan285..

Firefox ESR versions have superseded former levels

of macOS support, and are heading to cease ESR

support in macOS 10.12, 10.13, 10.14. Catalina 10.15

now at risk of becoming marginalized further.

Brave browser no longer has support for El Capitan

except for a few items that may or not see updates.

One of my iMacs topped out with 10.11.6 El Capitan

and an older Brave, kinda sorta still works, barely.

A few of my five Macs, capable of Catalina 10.15.7 are

held back; due to nostalgia, and older 32-bit software

from third-party vendors, some of that free by maker.

The obsoletism moves forward as threats to our data

may increase; however some settings in Mac KeyChain

'self-update' might be able to continue in limited cases.

Some web page owners have passed by older macOS

however Firefox "Extended Support Release" now is

at version 115.10 for Sierra, High Sierra, and Mojave.

~ As the last update 115.15.0 ESR will be in Sept2024.

[Firefox 116.0+ later requires macOS 10.15.7 minimum.]

https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

ESR/Enterprise: https://www.mozilla.org/en-US/firefox/enterprise/

..The later working supported Firefox gets pushed to run ESR,

then for an expected or theoretical three years, and dropped.