See:
http://www.archicadwiki.com/ArchiCAD%20versions
Near the bottom is a table of what versions each version of AC can read and write. As TMA says, 11 can save back to 10 (12 to 11, 13 to 12 etc.)
When back-saving, not only would you lose any info related to new features, if utilized in the project, but library parts cannot be saved backwards or opened in older versions.
Sorry,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB