That's a problem earn.com could have solved, but unfortunately only ICO promoters ended up using the platform, I think
I figured it was just spam similar to what this thread refers to because I only ever get crypto-related emails from it. If it wasn't for the more-premium-than-most domain I'd have already auto-spammed it
Edit: Naturally seconds after asking I find an email from Angel List saying 21.co rebranded to earn.com.
As it turns out I'd been added or at some point had added myself to an "Airdrop" list, which is a list of people interested in cryptocoins. Removed myself from that and unchecked "Receive emails for tasks below your contact price?" should finally stop getting emails about dodgy new cryptocoins. Successful rubber duck debugging