I got this email and it was not exactly easy to turn it off considering I hadn't logged in to my Airbnb account in a while. Perhaps a better solution would have been to provide a click within the email that would allow you to turn it off from there.

Or maybe even make the project, leave it off per user by default and include directions on how to opt in?

There is a direct click on the email that leads you to the setting page. I am using a gmail client, so not sure if the link doesn’t appear in other emails

Yes. There is a link in the email that leads you to the settings page but I hadn't logged in a while so after entering my phone number along with the code they sent me I also had to enter a code they sent to my email. My point being that clicking the link in the email should automatically turn off the "feature" I shouldn't have to log in.

My experience as well.

I don't read corporate newsletters from services I use twice a year, do you?

I always unsubscribe from corporate newsletters, so I don't get a whole lot of them. And in any case, this wasn't a corporate newsletter.

I delete any mail from Airbnb, Uber, etc that isn't a receipt for a service they provide.

As do most people.

How would you want them to notify you of a substantive policy/TOS/etc change in order to ensure you see it?

Alert me on the web page the first time I log into my account after the policy goes live.

Bingo.

I just logged into my Airbnb account and there's a notification about this policy change. It links to a page with the same text as the email notification.

Notification text: "We’d like to use your information to help fight discrimination. Visit your Privacy settings to learn more or opt out."

When I logged into Airbnb to fix this opt-in non-sense, I saw no such message.