"Plain text" in many cases is a user friendly name for text/plain MIME type – it's a text without special formatting (no markdown or markup tags, no special syntax, no reserved words etc.).
You can create many text type files in notepad, f.e. bat, cmd, html, xml, md, c, cpp, css, js, prg. You can even edit RTF documents, CSV database files, SVG pictures or components of epub/docx/odt/xslx/ods files. All of them are text only, but they contain formatted text, none of them is text/plain.
Test the lastest prerelease version: https://www.sumatrapdfreader.org/prerelease.html
It can display (and print) txt files, but it's stil far from perfection: it doesn't properly break long lines on white spaces and may improperly show accented characters in ANSI encoding (it seems to use html-oriented parser, so it may expect some ISO-8859 charset instead).