1) I do clean installs when I update macOS, so the Library folder is created new each time. Which means that archived messages only go back to that point. (I browsed via Terminal and used the command line 'open' command to view them.)
I do retain old ~/Library folders and it's possible to use the same technique in finding messages there.
2) But, and here's the big limitation: at a quick glance, they're simply individual messages, by date. There's no way to follow entire threads. I suppose it would be possible to write your own program to organize these. But the point of the article is that Apple should make it easy. Your method isn't easy.
Edit: response to sibling comment. I'm still on Catalina and the above works. I have no idea about Big Sur.
It does have some important limitations.
1) I do clean installs when I update macOS, so the Library folder is created new each time. Which means that archived messages only go back to that point. (I browsed via Terminal and used the command line 'open' command to view them.)
I do retain old ~/Library folders and it's possible to use the same technique in finding messages there.
2) But, and here's the big limitation: at a quick glance, they're simply individual messages, by date. There's no way to follow entire threads. I suppose it would be possible to write your own program to organize these. But the point of the article is that Apple should make it easy. Your method isn't easy.
Edit: response to sibling comment. I'm still on Catalina and the above works. I have no idea about Big Sur.