Microsoft are trying to standardise on MSIX as the defacto packaging and deployment format for Windows.
In the past ther release of Office has been seen as a confidence boost by customers, e.g. Office 2000 was an early MSI app, Office 2013/2016 were released dual MSI but later also C2R (App-V derived), and we'd therefore expect M365 desktop apps to follow suit and switch to MSIX.
There was talk back in 2021 that the Office team were looking at reworking the Office suite to be MSIX based but there's been no updates since - Re: Microsoft Office add-in deployment using MSIX - Microsoft Community Hub
