COM and VSTO add-ins aren't supported in the new Outlook on Windows :
The new Outlook on Windows aims to unify the extensibility experience across all Outlook platforms. To provide a more reliable and stable add-in experience, VSTO and COM add-ins aren't supported in the new Outlook on Windows. To ensure your add-in continues to work in the new Outlook on Windows, you must migrate your VSTO or COM add-in to an Outlook web add-in.
The major issue for us is that the ribbon access and customization is somewhat non-existent in Outlook web add-ins. We can only have in the manifest :
This is way too limited. We need the Ribbon API currently only available in Excel to be able to enable and disable add-in commands programmatically and create contextual tabs. We also need more controls like toggle buttons and checkboxes. All these things are available in COM and VSTO add-ins but not in Outlook web add-ins, which renders the migration impossible.
At this point of time in Outlook web add-ins, we can't even have a simple button control or menu item that enables a custom add-in feature with a label "Enable this [feature]" and make this button / item appear enabled when the user clicks it since there is no way to dynamically change the label to display another text such as "Disable this [feature]" and there is no toggle or checkboxes buttons available either.