There’s a bug in Outlook for Mac 16.58 where the switch to turn off New Outlook is missing. This is especially a problem because at launch it encourages users to opt into the New Look without telling which features (including some required by SpamSieve) will be lost by doing so. The release notes claim that the switch has been restored, but I found that even in the 16.59 beta the switch is still gone. You can, however, turn off New Outlook by entering this command in Terminal:
defaults write com.microsoft.Outlook IsRunningNewOutlook NO
and then restarting the app.