I use Outlook 2016 to access Yahoo email, using POP3. Recently Yahoo tells me this is a "less secure logon". If I change the Yahoo setting to not use less-secure, then Outlook will not connect to yahoo email. Does this mean Microsoft Outlook 2016 is quite secure? Or is Yahoo saying not to use Outlook?
Yahoo, Gmail, and other email providers switched to OAuth 2 and turned Basic Authentication off by default. When you attempt to add the account to Outlook, you are unable to authenticate because Outlook uses Basic Auth, not OAuth 2. When you log into the website to enable basic authentication, you are warned that it is a less secure login.
Yahoo, like most webmail providers, would like you to use their web interface so they can show you ads, but they are not saying not to use Outlook (or other email clients), only warning you that the basic authentication method is not as secure as OAuth2 and could make it easier for someone to break into your email account. Once you are authenticated, you'll use SSL/TLS to send and receive mail.
If you enable two-step verification (also known as multifactor authentication), you can turn off the 'use less secure login' setting but you will need to use an app password in Outlook. I recommend using two-step verification and disabling less secure logins. Yes, it can be annoying (especially if you don't always have your phone handy), but two-factor verification is more secure.
For more information about Gmail accounts and the less secure app warning, see Gmail in Outlook: unable to connect to email. When Google's 2-Step Verification is enabled for Gmail accounts, you don't need to allow less secure apps, you'll use an app password to log in with instead.