sub to txt conversion is most likely searched by users that want to edit subtitles with text editor and think they need to convert the subtitle file to text. This is not needed, as .sub files are already specially text files, they just have different file extension.
If you really need to, you can change the extension from sub to text, but text editors can open the files regardless of their actually file suffix. Just keep in mind that if you want to edit subtitles, it might be best to use specialized subtitle editor instead text editor.Open SUB file Open TXT file
A free subtitle editor