TLDR it replaces an expired certificate, no software is being "updated" here. Wake me when old versions of OS X can access the App Store again.
What versions can't access the App Store anymore? I've tried Catalina recently, and that still worked fine, but it only stopped getting security updates in 2022, so it's only been a few years. Also, I've barely ever used the OSX/MacOS app store anyway, and from what little I've heard from other people, it's not really all that great nor popular a place to get your software from.
Of course this is completely opaque to people who have to do this, it just ends up prompting you to login and things like that.
I think newer MacOS avoids this stuff by not having OS updates be linked to the App Store