So, I have looked into this, and the folks at Microsoft are misleading everyone when it comes to New Outlook (aka Mail).
The IMAP function has been removed, so you won't be able to use anything other than 365 or Outlook (personal accounts) on there.
Mail has been discontinued, and you can no longer roll it back, no matter how hard you try. Unless someone knows where to get an old version that can be downloaded as a standalone app, there’s no way to revert. If you try and it doesn’t work, you’ll lose any current IMAP accounts that sit outside of Microsoft—so be warned.
Microsoft wants everyone’s data, so for some users, you can get a code to allow Microsoft Cloud to act as an interface between you and your non-MS accounts.
So your email will go:
Client (desktop app) → Microsoft Cloud → Your email provider.
Even basic functionality like rules, which normally sit on the client (desktop app), will no longer work on none Microsoft accounts.
Microsoft has made New Outlook unusable to force you to either pay for the full version as a perpetual license for one PC/laptop or get a monthly subscription.
To answer your question:
If your emails were set up as IMAP accounts, with luck, you should be able to get them back, depending on your email provider, by accessing the web version... I lost a bunch, but I use a cheap email provider.
If you set them up as POP3, they are gone. They are in there somewhere but you can't access them. Hopefully some clever tech can figure it out.
As far as I can tell Microsoft accounts are not affected, so if you were using anything from MS you should be fine.
If there is a Mod on here from Microsoft I would be happy to be corrected.