I'm guessing imapsync could be used to transfer email into Gmail to replace this POP client feature that is being removed, as long as it can sync into a folder.

Maybe... but at that point you're self-hosting a service/app instead of relying on google to pull your other account into gmail. At such point it may be best just to use your vanity email first, and yoink out your gmail into that account.

Hey, first off, thanks.

How do you go about that? The documentation says:

Q. Can imapsync be used to maintain and restore a local offline copy of a mailbox, eg for backup purposes, using Mbox or Maildir format, so that if the server fails, then the mailbox could be reinstated?

R1. No. Imapsync plays with IMAP servers only.

Ah yes, I run a local IMAP server (Dovecot).