I have an Outlook Add-in that makes use of the displayNewMessageForm, it works on Outlook Web and Outlook Full Client, but does nothing on the mobile client (and debugging on the mobile device for Office addins is next to impossible).
I saw this previous question:
Office.context.mailbox.displayNewMessageForm not working in mobile
Where someone said "Outlook Mobile don't support displayNewMessageForm and displayNewAppointmentForm as of now. The support for display* APIs are in pipeline and will be available soon."
That was back in 2018. I looked at the Microsoft Docs, and many methods have a note saying "This method is not supported in Outlook on iOS or Android."
This method does not.
So does anyone know if this is supported in Outlook Mobile clients?
It is not supported yet. APIs introduced in requirement set 1.6 or later are not supported by Outlook Mobile nowadays. Take a look at the Unsupported APIs section of the Add support for add-in commands for Outlook Mobile article.
As soon as the requirement set 1.6 is supported on mobile clients such as iOS and Android you will get the function supported there.
You can find the supported hosts in the Outlook client support table. As you can see the highest requirement set is still 1.5.