MMRS asks for one thing: an email address, typed in by the person who owns it, with a box ticked to agree to the emails.
The address, which list it joined (the digest, or a follow of one aggregator, and which), the date, and the page the form was on. Nothing else: no name, no IP address, no tracking.
In a private store of the site's host, Vercel. Private means no web address serves the records; only the site's own signup code and its operator can read them. Addresses are never published, sold or shared.
The emails the person asked for: the digest, or notice that a followed aggregator's record changed. Every email carries a one-click unsubscribe, and an unsubscribe removes the address from that list.
Anyone can ask for their address to be deleted entirely, from any email, and it is.