Did I miss the announcement of the end of support for SMS messaging from Exchange Online?

Hi Adrian Hyde 

Searched - nothing to say that the functionality has been deprecated. 

Follow the link Victor provided (https://outlook.office.com/owa/?path=/options/textmessaging) and it will take you to the Text Messaging section under General mailbox options

Select Set Up Text Messaging and it will give you the options

AFAIK on 365 this is for Web App Only, so wouldn't likely appear in the ribbon.

https://support.office.com/en-us/article/text-messaging-notifications-in-outlook-web-app-4154bd15-4f2b-4854-8297-1ae55649f158

Hope that answers your question.

Best, Chris