It appears (?) that these windows may only be available in the INT library.
Some confusion over this as a 'new feature' vs who will actually see it in their library... maybe GS can respond.
The description seems (?) to be only in the (online) Help Center content for the INT version:
http://helpcenter.graphisoft.com/archicad-18-int-reference-guide/archicad-18-new-features-guide/libr...
The USA/NA 18 Refererence Guide PDF does show these Multi-Sash Windows on page 114... and unlike, e.g., the "New Sectional Doors" on page 113 that are tagged in italics as "selected markets only"... the multi-sash windows don't have a disclaimer.
One of the forum moderators
AC 27 USA and earlier • macOS Ventura 13.6.9, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB