Ask HN: Is there an email service that allows plaintext and respects my privacy?

Privacy there are some, plaintext-only. I don't know. You may just host a email-server yourself if you want to get what you're looking for.

I would recommand Fastmail (fastmail.com). Not free, but they respect your privacy and there are options to compose and read email in plaintext.

I'm curious why you don't want to use HTML.

Tracking is one reason.

Every time I get an email saying "We noticed that you haven't been opening our emails", it makes me smile.

FastMail also has an option to filter out tracking links from HTML emails (from everybody, or just from unknown senders), while still giving the ability to see pretty HTML formatting if you want it.

outlook.live.com has a plain text setting / bottom arrow on new/compose page

I wrote one. It's a side project, and dear to me, and private but you can use it if you'd like. Email in my profile.