I wouldn't expect RTF to be supported by Excel. RTF stands for "Rich Text Format" and is basically a "Word Processor" format. You're saying that you must save "spreadsheet data" (rows and columns) to RTF. The reason you can get away with such an activity is that modern "word processor" documents can contain objects, like images.
Really, Spreadsheet data belongs in a Spreadsheet format. So rows and columns can go into .xls (Excel Spreadsheet) format (or .ods format, the Open Document Spreadsheet favored by LibreOffice). For convenience, modern software allows you to include data from other formats, which is much better than the old ways of doing things (which is to read a reference, and then open up some other software to be able to see the related data). However, there's no technical advantage to complicating software by trying to directly save in a format that is intended by other software (like a Spreadsheet trying to save in a Word Processor format), nor is there a large advantage to the average user's "user experience". So, I'm not expecting such a feature to be added.
In fact, RTF is not a format that I see people talking about much. I used to really like the RTF format for its apparent compatibility advantages. (However, I believe RTF is actually a bit of a loose standard, with lots of specific details being left up for individual software to make its own interpretations. This may make it easier for a software manufacturer to have some support for an RTF format, while making it more challenging to try to have all the software agree with every nuance. The result is that an RTF document might have some details that are treated differently by different software. For instance, I've found that Wordpad may open a document with different margin settings than Microsoft Word.) With quite a bit of non-proprietary software (like LibreOffice) providing better and better support for Microsoft Word's .doc format (or even the .docx format), the compatibility advantage of RTF seems to be getting less and less significant. I think it is more likely that Microsoft might drop support for RTF (which they did with the Windows Write .wri format, in Windows XP Service Pack 2) than for Microsoft to expand that format. I believe I did read that RTF is considered to be rather unspectacular from a technical point of view, so there may not be a lot of widespread desire to see its use expanded. Therefore, I suggest not placing much hope in the idea of Microsoft increasing support for this format in newer versions of the software.