These days iPad's Mail app will let you set an Archive email folder for any IMAP account (iCloud, Yahoo, and others). It's under the Advanced settings in the account; where you can also set emails to be deleted (trash) or archived by default. You can, of course, use the Move option to place an email in any existing folder you like.
Yes, Gmail used to be the only one with the archive option. Then again, it used to be that Gmail archived your emails off the iPad no matter what your setting were. At least that was my experience a year or so ago. I haven't tested it lately.
Personally, if an emails is important enough to save I don't like it being stored just in my email account. Which is why I either print them as PDF to a receipts folder on the iMac (using Printopia), or forward them to Evernote; whatever makes the most sense to me at the moment.
I also do occasional archives and exports of Evernote, just in case it goes away. Not that it looks like to happen any time soon. They are doing well.
All of this gets backed up on an external hard drive, automatically via Time Machine, and to an online offsite backup via BackBlaze.
Being a formar IT tech, I can be a bit paranoid.