According to a Tech Community post from Microsoft, support for Microsoft 365 apps on Windows 8.1, the three-year Extended Security Updates (ESU) paid program for Windows 7 and Windows Server 2008 R2 are set to come to an end on January 10, 2023. Therefore, if you are currently using Microsoft 365 apps on any of these operating systems, then you need to act swiftly and transition to an OS that is currently in support and shield yourself from security breaches by hackers.
Microsoft has further indicated that they will remain committed to providing security updates for the length of these ESU programs, though "Microsoft 365 Apps is not currently supported on Windows 7 or Windows Server 2008 R2". Furthermore, any additional installations of Microsoft 365 Apps will be blocked on Windows 8.1 devices. This shows the importance of transitioning to an OS that is currently in support as you will continue getting security updates as well as new installations. Be sure to also check out our post which highlighted new features that made it across Microsoft 365 apps in June this year.