Poor quality articles. They use flexbox and provide no fallback for older browsers. They could use at least non-responsive layout for desktop resolution for older browsers. Probably the reason why they didn't do it is lack of knowledge of CSS.
Also, if I remember correctly, the default browser in Windows 7 is IE9. It makes sense to support default browser in the most popular desktop OS.
Some of older browsers, released in 2012-2014 support flexbox only with vendor prefixes, but the article doesn't has rules with prefix. As frontend devs tend to copy the code without much thinking, we cannot expect that they will add the prefixes or fallback code themselves. So it will be author's responsibility for web sites being less accessible in different browsers.
> Some of older browsers, released in 2012-2014 support flexbox only with vendor prefixes
Again, these browsers represent a tiny fraction of the total market. Firefox and Chrome auto-update by default, so there is no reason that someone should be on a version from 2012.
These days it simply doesn't make sense to have a fallback for flexbox unless you're specifically targeting older versions of IE. (And even then, I've had some luck getting flexbox layouts to work on IE 10, even though it's more effort.)
Always get data about browser market share from your client if they have it, otherwise you're simply in for an uncomfortable surprise one stats start coming in.
You can choose to do that if you want, but I think your opinion about the "spirit" is a minority one.
The spirit of HTML was not adequate to meet requirements, so the spirit has been upgraded. Pray all you want, it's going to be upgraded further.
I wouldn't be scared of using flexbox even if you need to support IE11. I'm not sure where caniuse's dire warnings come from. (I have run into different flexbox behavior between FF/Chrome about as often I have run into flexbox problems with IE11; I generally couldn't tell you which of FF or Chrome is correct/incorrect in those cases).
I've in the past done flexbox without too much trouble even on IE10, but I am not myself concerned with IE10 anymore.
https://inclusive-components.design/
IE11 is meant for supporting legacy sites and Microsoft doesn’t recommend consumers to use it. It make sense to work on Grid and Flexbox for new sites.