That isn't always true, depending on your choice of web server. You can use mod_rewrite rules in Apache's .htaccess files so if your generator is aware of previous URLs for given content it could generate these to 30x redirect visitors and search bots to the new right place.

Off the top of my head I'm not aware of a tool that does this, but it is certainly possible. It would need to track the old content/layout so you'd need the content in a DB with history tracking (or a source control system) or the tool could leave information about previous generations on each run for later reading. Or it could simply rely on the user defining redirect links for it to include in the generated site.

Of course if you are using a static site generator for maximum efficiency you probably aren't using Apache with .htaccess processing enabled! I suppose a generator could also generate a config fragment for nginx/other similarly though that would not be useful if you are publishing via a web server where you do not have appropriately privileged access to make changes to that.

You can do 301s statically, by generating whatever your particular version of an .htaccess file is in place. Or, you can generate the HTML files with the meta-redirect header in place.

The DNS is obviously an issue, but that's not really relevant. The article is advocating for URLs not changing. It's not saying that they mustn't change, just that it's really cool for everyone if they don't.

I know it's 2020 and all that, but sometimes you don't need 20 MB of minified JS to achieve something: https://en.wikipedia.org/wiki/Meta_refresh

Netlify allows dead simple redirects, and so do most other static hosting platforms.

When you change the structure of your urls, you can generally generate redirect rules to translate old urls to the new structure. Or run a script to individually map each old url to its new one.

Note: I've never done the layter for more than a few hundreds urls, I don't know if it scales well for a very large site

This is a poor analogy. Perhaps “I’m a librarian for a library with thousands or millions of users, and when I rearrange the books, I don’t leave sticky notes pointing to the new locations”