I ran xmlstarlet using ipe.dtd on an ipe document with notes.

/opt/homebrew-cask/Caskroom/ipe/7.2.2 $ xml val -d ./Ipe.app/Contents/SharedSupport/doc/ipe.dtd -e ~/Documents/talks/keynote-micw.ipe

The ouput includes:
keynote-micw.ipe:22580.0: Element page content does not follow the DTD, expectin g (layer* , view* , (group | image | use | text | path)*), got (notes layer layer view image text text text text path path path )
...
~/Documents/talks/keynote-micw.ipe:22581.0: No declaration for element notes