Rocketmail is one of the oldest webmail names, long since absorbed by Yahoo and that history is the key to both accessing and backing it up, since it lives inside Yahoo's system now:

The access reality, Rocketmail as Yahoo: Rocketmail was acquired by Yahoo many years ago, so rocketmail.com addresses are Yahoo Mail accounts, accessed through Yahoo Mail at mail.yahoo.com or login.yahoo.com with your rocketmail.com address and password, and managed through Yahoo's account system. So accessing it is accessing Yahoo Mail, and any Yahoo Mail guidance applies to your Rocketmail address directly, which resolves the unsure of its status question, it is a Yahoo account wearing an older domain.

The client access for backup, over IMAP: since it is a Yahoo account, it uses Yahoo's IMAP settings, imap.mail.yahoo.com port 993 SSL and smtp.mail.yahoo.com port 465 SSL, and critically Yahoo requires an app password generated in the Yahoo account security settings for mail clients rather than the normal password. Add the rocketmail.com account with those settings and an app password in any mail client, and it connects like any Yahoo account.

The backup, once connected: with the account in a client like Thunderbird over IMAP, the mail syncs down and you back it up by copying it into Local Folders for a local copy independent of the server or exporting to mbox with the ImportExportTools NG add-on for a portable archive, the same durable local backup approach as any webmail. Alternatively Gmail's import feature or Yahoo's own account tools can pull the mail elsewhere, but the client plus Local Folders route gives the clean portable backup.

The preservation urgency worth noting for an old account: an ancient Rocketmail address you rarely use is worth both securing, current recovery information and a strong password since old dormant accounts are targets and backing up, since the mail represents years you would not want to lose if the account ever lapsed or was compromised. Getting it into a client and copying to Local Folders or mbox now converts an old vulnerable single copy into a preserved one, the sensible move for any long held account you value but rarely tend.