It's been gone-as-a-default for a few months now, but for a long time, "Classic Outlook" aggressively offered a one-toggle upgrade path to "New Outlook", and it got used.

Sure, "New Outlook" on Windows is still entirely broken (including stuff like, oh, re-ordering mailboxes), but on Mac it's actually quite usable, and even allows you to switch off the IMAP proxying.