I'm not sure what the reasoning was for changing the content but the cause of the general change is actually really cool. The language reference pages content is now generated from a GitHub repository so it's super easy to propose changes and easy for the Arduino team to merge those changes and push them to the website. They even have an "Edit" link on every page now.
Here's the repository: GitHub - arduino/reference-en: Editable source for the Arduino Reference
So you are welcome to submit a pull request for adding that content back in. It looks like you can still access the old content by modifying the URL a little. For example, the new attachInterrupt() URL is:
The old URL is:
https://www.arduino.cc/en/Reference/AttachInterrupt/
Note the change in case.
I don't know how to access the old reference home page.
I suspect there will also be some regressions of recent improvements made to the old reference pages because they started on this new reference system years ago with the reference content at that time but then lagged super hard on the project until a few months ago when they started working actively on it, but without redoing the scrape first.
They're also using the same system for translating the reference to other languages:
Supposedly the plan is to do the same with the library reference pages but that content will be stored in each library's repository.