Presentation is the second most important thing to get right in your CV next to the actual content.
While there are CV templates that take the whole presentation thing too far, sending your CV as a text file is going much too far in the other direction. Even if you all you want is a clean and clear document, a text file won't provide the options to set things like spacing or columns that actually enhance readability (which is presumably your end-goal with your no-frills CV).
There are no options to emphasise the important parts of your CV. Even bold text formatting or subtle colour changes that catch the eye but aren't distracting cannot be done in .txt files.
What you will be left with is a long, boring-to-read list of things that you want to sell yourself with. Your CV is supposed to be your sales pitch, your advert. What a .txt file will say to me is that you might be talented but you clearly couldn't give many ****s about this job.
Make your CV in a word processor, then export as a PDF to send to companies
PDFs have a few distinct advantages over just about every other format going:
They are supported by just about every word processor out there. Word, LibreOffice, hell even LaTeX will all make PDFs without much issue
They are supported by just about every modern system going in terms of reading them. PDF support is integrated into all modern browsers, and almost all modern OSes have some inbuilt ability to read them perfectly. Which brings me to my next point
The document you see on your screen will be exactly what they see. This may not seem obvious but it can be a problem for documents sent in DOC/DOCX formats. Formatting discrepancies can happen with different versions of Microsoft Office but is especially prevalent with word processors that aren't Microsoft Office. With PDFs, you can trust that the formatting will not be messed with.