Rich Text Documents usually have rtf extension, but some programs save them with rtx extension instead. Any program capable of rtf to txt conversion will ne able to perform this conversion, but you may need to rename the suffix first from rtx to rtf.
Convert Rich Text document to Simple text file.
We have found 2 software records in our database eligible for .rtx to .txt file format conversion.
Microsoft Windows software - convert rtx to txt on Windows
A free source code and text editor for Windows
Convert from rtx file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned rtx to txt. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert rtx to txt on OS X
The default text editor in Mac OS X