Hello everyone and thank you for taking the time to request this feature. As part of our investigation on adding this capability, we ask that you please help us gather additional information on how you use Pin. This helps us to prioritize and design the feature to be exactly what you need to get more done!
Please use the below link to access a short survey about Pin:Office 16 user commented
This was posted in 2016... How is this not a feature yet?
The Outlook application needs to have a pinned feature and the lack of one here (but other office products have it) is a joke.