Using older versions of Outlook for Mac in business environments
What that means is that Outlook.com will require Oauth2 ("Modern" Authentication) just like Gmail and Yahoo already do. If your current email client will natively work with either of those, it will work with Outlook.com after the switch-over, though you may need to re-create the account. If your email client does not do Modern Auth, both Gmail and Yahoo will supply (allow you to generate) and require you to use an Application Specific Password instead of your normal password. Outlook.com also has a generator for these passwords. Please see these support articles:
How to get and use app passwords
Sign in to apps with your Apple ID using app-specific passwords