The idea in the link is that one transfer the System Root certificates from another (more modern) Mac to which the customer should already own. In theory I guess we as a email provider could continuously buy the latest version of mac and use that as the transfer host (hopefully do not transferring any account specific certificates). It must work without any corner cases (not overwriting anything on the customer end), the script need to work in future macos versions, and the assumption is that the first group of certificates is the relevant ones, and that it doesn't break anything on the customer end.

For those who has not read the link, this do not update the System Roots keychain since users are not allowed to do that in macos. Instead this adds a trustRoot certificate. A nice trick. One of the comments say that it didn't fix the problem for them but who know what their situation was.

Looking at the compatibility charts, everything from 2015 on-ward supports Monterey (2021, the latest version). The last laptop that couldn't update past El Capitan was 2009. As far as Macbooks (non-pro), last one stuck on El Capitan is the 2009 model as well.

And it's not even that Apple broke anything on the 13 year old laptop, it's just that the rest of the world moved on and they're no longer fixing it.