Since Illustrator 9 an AI file is a PDF until Illustrator appends its private data section. If you leave Create PDF Compatible File checked, any PDF reader can open the artwork, and the file arrives at a printer as something a human can inspect.
Uncheck it and the file shrinks, but the reader outside Illustrator sees nothing. Most tickets we see titled “AI file won’t open” are this one checkbox.
Keep the compatibility flag on when the file leaves your machine; turn it off for internal working files where the size saving matters.
Where this came from
Notes like this one come out of the file library rather than out of a release schedule. The pages linked above are the entries that were being worked on when the note was written, and those entries are rebuilt with each release of the toolset, so a note can go out of date in a way a format page does not.
When that happens the note stays where it is and gets a correction at the bottom rather than being rewritten, because the point of most of these notes is the order things were tried in. The editors who write them are the ones who maintain the catalogue.