Pretty much every Word feature is supported by rtf. Microsoft owns rtf and updates the format every time a new version of Word comes out, specifically so that it supports the new features. This happened early this year with the release of Word 2007 on PCs, for example.The problem, of course, is whether this is included in RTF or is a proprietary MS thing in Word. While tracking changes would be useful in NW, it would be 20 times more useful if the NW tracking could show up in MS Word and Word tracking could show up in NW.
Anyway, rtf has supported tracked changes since at least 1998.