It's interesting RTF is discontinued when it is still the default format on the included word processor in Windows. I checked WordPad on Windows 10 and it still saves to RTF by default, though it supports docx and odt as well.
Unless they changed something the last couple of years, WordPad is just a wrapper around the Windows rich edit common control[1], which as its name implies, is based around RTF[2].
Microsoft is famously backwards-compatible, and support for RTF is still present in those programs which originally had it, like Write, the included word processor in Windows since Windows 1 (replaced in Windows 95 with WordPad, which retains its features). But is RTF usable for its original purpose – interoperability? Can you open a reasonably complex Word document and save it as RTF, and then open it in LibreOffice? Can you save an RTF document from LibreOffice and open it in Microsoft Word without loss? Is Microsoft working to maintain RTF interoperability in order to promote cross-program document compatibility? No. To quote Wikipedia, “Microsoft has discontinued enhancements to the RTF specification. New features in Word 2010 and later versions will not save properly to the RTF format.”¹