They'll change. Maintaining backwards compatibility with third-party training material is the least-useful form of maintaining backwards compatibility.

This is forever the problem with documentation: it checkpoints a description of a system at a point in time.

You can make an extremely valid similar argument regarding C++ tutorials written in 1995, but the end-response is the same: "Update your sources, learn the new thing, and most importantly don't assume anything computer-related that is more than 5 years out of date is relevant, especially for something Internet-related."

Okay but unless other browsers make the same change now you have two sets of information and now users need to know their underlying browser's engine too?

That's going to be up to the other browser vendors. Some would argue that multiple approaches to these problems is a virtue of the ecosystem. (I'm not sure I'm one of them, but if one accepts that premise than "This documentation varies based upon what browser you're using" is an expected side-effect).

That advice has been deprecated for years and has never been sufficient. The reverse is true now: the browser will warn about an insecure connection.

That advice didn't seem to be a good idea.

It's still true of virtually every browser other than future versions of Chrome and possibly browsers based on Chromium.