I thought plain text meant no syntax highlighting... I only stated the latter to make it clear what I was running into. Plain text also includes any rich text formatting like Bold or Italics etc. I just want plain text as plain as you can get.
I may look at any sort of text or file, that may or may not contain any type of programming syntax... I don't want the editor to do anything but plain text, so I'm hoping there's a better solution than disabling every possible programming language bundle in the editor...
No. Plain text is just text that does not store any information in its file format about typeface (though it *does* store information about encoding---e.g., UTF-8), and which cannot render graphics and the like. Text editors and IDE's (e.g., Emacs, Vim, Sublime Tex, Textmate, etc.) all operate in plain text mode in the sense that they do not write formatting information back to the files that they are working in. This includes syntax highlighting, and the display of any graphics referenced in, say, the <img> tags of your HTML file. Notepad is not
more plain text than these other editors, it's just a more barebones editor that lacks things like syntax highlighting, auto indent, auto-completion, in-line preview, etc. etc. that make working with various programming and markup/down languages easier. In either case, though, you are working with plain text in terms of what you are actually saving to a file, and then that file either renders to an output format when viewed in an appropriate viewer (a web browser, in the case of HTML) or is otherwise compiled in some fashion or other. This is very different from rich-text, a-la .docx and .pages, where there is no separation between content and formatting.
In terms of editors that come preinstalled on the Mac without any bells and whistles, I think Nano would probably fit the bill:
However, it is a terminal app, and there is a learning curve if you're coming from something like Notepad. There's also Sublime Text and Atom, which don't come preinstalled, but I'm fairly sure (in Sublime's case, completely sure) that they allow you to toggle syntax highlighting on and off without having to mess around with bundles---although if you don't want syntax highlighting, there'd be no harm in removing said bundles from Textmate.