I think this “Simple Text Widget” need some improvements. While it is “text plain” but it’s really not, per se. The problem is when you paste texts from 3rd party such as Microsoft Word or from any website pages, it does have styling (inline, etc.) and there was no way to remove even if there’s no source code it clean it up.
My suggestion is to add/update TinyMCE plugin that does unformatted text.
[paste\_remove\_styles] If true, removes all style information when pasting, regardless of browser type. Default is false.