Starting January 10, 2023, Office 365 will completely stop supporting Windows 7/8.1/2008 R2
Microsoft normally provides regular updates to Microsoft 365, which is essentially always supported as long as the update is maintained, but there are exceptions. That's Windows 7(specifically SP1), Windows 8.1, and Windows Server 2008 R2 versions, which will no longer be supported by Microsoft 365 (including other Versions of Office 365).
As for why January 10, 2023, because Windows 7 and Windows Server 2008 R2 have long since stopped supporting it, Microsoft still offers ESU extension support to paying enterprise customers, and that's when the extension ends.
It doesn't make much sense for Microsoft to continue providing Support for Microsoft 365 after the OS has completely stopped supporting it, since Microsoft was primarily focused on enterprise customers.
According to Microsoft's announcement, Windows 7/8.1/Server 2008 R2 will not receive any security updates until January 10, 2023, even for paying ESU users, Microsoft 365 will not receive any security updates. Users must upgrade to a supported operating system if they want to use the new version of Microsoft 365 or get support for security updates.
Currently supported operating systems are Windows 10 LTSB 2015 Enterprise or Windows 10 20H2 Consumer/Enterprise and Windows Server 2016.