So why does it only affect btconnect addresses and not btinternet addresses?
I presume it's because btinternet.com addresses can be set up as IMAP whereas btconnect.com addresses can only be set up as POP3.
A helpful chap from BT Business pointed me towards https://outlook.office365.com/mail/ where I was able to login with my btconnect.com details and then go to settings to divert emails to another email address. Emails also stay there too.
No, I don't think it's an IMAP/POP3 thing, it's an alleged insecure app (Outlook) thing.
Btinternet addresses can still be used with Outlook both for IMAP and POP3