Update: I just tried today and it FINALLY worked for "synced" Passkeys and creating it within 1Password!! I no longer have to use that stupid "Microsoft Authenticator" app anymore!! So it appears within the last 3 weeks or so (since the last time I tried), Microsoft finally enabled the ability!!
I have two Microsoft Exchange accounts (two different companies). One company it gave me the option of "Security Key or Passkey" (did NOT say "(Preview)" on it, so it appears it was out of "preview". But after I created it, I went back to get a screenshot and that option then disappeared, leaving only "Security Key" or "Passkey in Microsoft Authenticator"). The other company it didn't have that "Security Key or Passkey" option, so I had to use "Security Key" and then selected either "USB device" or "NFC device" (both worked with 1Password).
For the "Security Key or Passkey", I was able to set it up on an iPhone using 1Password as the passkey provider.
But for the "Security Key" option, I had to do it via a PC (I was using Firefox, if it mattered with the 1Password Extension set as Firefox's default password manager). If I attempted in iPhone using the "Security Key" option it asked me to connect the device to the phone, it wouldn't prompt to save it in 1Password, even with that being my iPhone's default Passkey provider (I am not using iCloud Keychain).
NOTE: it was giving an error at first, if "Enforce Attestation" was enabled. But when I turned that off (I am an admin for both companies), and also turned off "Enforce Key Restrictions", then it worked. This was even when I had the AAGUID for 1Password in the "allowed" list, but the warning still said that "my organization" didn't allow it.