a) there is no 'unlimited storage' tier, and certainly not for $0.99/month.
In the US, $0.99 gets you 50GB of storage.
Either way, I can almost guarantee that the email is spam, designed to trick you into follow a link urgently to 'protect your data', or some such.
To be clear, Apple DO NOT DELETE your data, even if you stop paying for storage. Bad things start happening (data may not sync between devices, backups won't happen, etc.), but existing data will remain intact. Therefore the threat that you're about to lose your data is a huge red flag.
Even then, if you stop paying, you'll drop to the standard 5GB tier. There's no such thing as 'terminating an iCloud account for non payment' since the basic account is free.
Ironically, it's clicking links in the email is almost certainly going to result in you losing data, and a lot more.
I hope you didn't click any links in the email. If you did, go to: Keep your Apple Account secure - Apple Support