We routinely send email promos and have to follow up with an SMS to those who have not opened the email.
I know this can be done in Journey Builder with an engagement split but we can't predict which emails need an SMS message so Journey Builder is not viable.
As of now, all I can think of is to go to Tracking Sends > Email_Name > Opens > and Export the subscriber keys field of those who opened.. Create a new DE.. Upload those subscriber Keys.. Go to Automation Studio.. Create a Query to Join the Original DE with the new DE of those who opened by subscriber key.. and finally add that to the Excluded field in Mobile Connect.
It works but I would highly appreciate anyone with a better solution.