Plivo was hacked or sold user info

According to their privacy policy [1], they share your email with third party advertisers. You have to opt out if you don't want that.

[1] https://www.plivo.com/legal/privacy/#how-does-plivo-share-yo...

Probably the latter, wouldn't be the first time a company does that.

I'm signed up with Plivo but so far it doesn't seem like I've got any spam to that email (yet?)

Or the email was sent in plain text (typical) and an intermediate hop logged the address and made it available.

Or the matching address was generated algorithmically because if you're spamming you don't really care if most addresses bounce. Thinking about the days when I used to use bespoke addresses, they would have been pretty easy to generate algorithmically. And thinking about spam I've received, what appears to be algorithmic generation is not uncommon.

Or the Terms of Service you agreed to include sharing information with partners.

Good luck.