If I remember right their mac-thing was around 5 years old?
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.
My 2015 MacBook still runs absolutely fine.
For that matter, I'm considering (but probably not) buying a new iPad to replace my TEN YEAR OLD iPad that I'm still using every single day, as it can no longer update to the latest iPad OS.
But my 7 year old (soon 8 years old) laptop is as good as the first day I bought it.
IOW, if one is going to supply anecdotal data, probably best to make sure it's your anecdote, maybe?
The customer must be running some truly ancient version of Mac OS for this story to be true.
Some providers (email and others) can also continue to support deprecated/broken security for a very long time in order to not having to deal with customers using old deprecated software. 512 bits RSA public keys for example was used a very long time, long past where different standards and recommendation had a much higher minimum. Algorithm and key attacks are quite rare so companies can keep a low security standard for many years/decades before anyone exploits it. For customers that could be seen as a positive or a negative depending on perspective.