You can use HTML with plain text fallback. That's still the vast majority of marketing/form emails.

HTML works fine, and adds important features like formatting text and links. HTML without images is ideal, as images are often blocked (as you mention).

I prefer plaintext too, but for a marketing email (ie, "confirm your account") consumers will think it feels "off" or unpolished given what they are used to. It will likely lead to churn. Clients can opt to view plaintext if they prefer.

Edit: also it's a bit funny the page you link, useplaintext.email, uses lots of HTML formatting.