I use Office 365 ProPlus. As a licensed user, may I also use another version of Microsoft Office instead?
No, you may not, because if you use Microsoft Office as a service (cloud application) and not as an on-premises variant, no rights to an earlier version (downgrade rights) are available to you. Furthermore, a Microsoft Office cloud application does not give you the right to install an on-premises version of the software instead or in addition.
Source: Licensing brief_Microsoft_Office 365 ProPlus Subscription Service in commercial licensing_2020-04