I was very careful not to click on any links in the email.
I was not fooled even for a second. It was impossible for me to have run out of storage. You know why? Because I paid for a Proton Unlimited subscription a few months ago, and I have more than enough storage!
The fact that I got this email means someone got access to my email address, which makes it hard to narrow it down because I used to sign up for a LOT of services. I've started using Addy to create new aliases, but I really wish I had done that sooner, so I could see who sold me out.
Here's a lesson: always use an email alias, so that when/if this happens, you'll know who leaked your email address!