Outlook still used IE11 for some of its functionality up to Office 2021, until Microsoft recently patched it to use Edge. 2013 to 2019 still use IE11 for render

Outlook still used IE11 for some of its functionality up to Office 2021, until Microsoft recently patched it to use Edge. 2013 to 2019 still use IE11 for rendering some functionality inside Outlook. And of course MS Word (!) for rendering emails.