What else should I do? The list is double opt-in, every message includes a one-click unsubscribe link, full contact info for the organization is included, and I send text-only.

My reading is that in your comment they replied to, you said you require confirmation for unsubscription.

One click unsubscription is presumably what they want.

I require a button to be clicked to confirm. No entry, no JavaScript, nothing else. Just something to make a POST request because I receive GET requests for almost every URL I send out.

My experience is that every unsubscribe goes to a form w/ a submit button. Shitty ones make you type your email address. (Mine doesn't.)

I dont have a problem with that but it’s definitely a second click.

Just did a bit of unsubscribing and sydneytools.com.au, abc.net.au, squabblr.co, bundlehunt.com, and healingstreams.tv all one click unsubscribe.

I've seen a hybrid where you have a form with a button to confirm, but include JavaScript to auto-submit the form on load. For the crowd that has JS disabled, they can click the button, but otherwise it's one-click from the email.

No idea if this holds if/when the email crawler bots start executing JS on crawl.