We recently created a VSTO Add-In for Outlook 2016 using the Outlook 2016 Office Project Type in Visual Studio 2017 and was recently asked if it's possible to port our Outlook Add-In to mobile devices (IOS/Android).
We've done some research on whether this is possible, however not sure if this can be done, at least easily from reading this article.
https://learn.microsoft.com/en-us/outlook/add-ins/outlook-mobile-addins
Any help or guidance would be appreciated.
Microsoft Outlook app on mobile platforms is doesn't support VSTO add-ins. If you would like to bring some functionality into mobile apps you may consider to look at Office.js Add-ins. The resource you have provided talks exactly about this type of add-ins. You may get an idea if you want to invest into development of Office.js add-in by reading ... How are Office Add-ins different from COM and VSTO add-ins?