Why Swiftkey not visible in OxygenOS 3.1 Search?
Official Oneplus answer about the topic


Revert OS back to 3.0 over remote session


  Since this issue have occurred after an update then I would like to suggest that you undergo a remote session. This could be a compatibility issue thus I suggest that you revert back to the previous OS version where you are having no issues at all. 



This choice is one viable solution because the normal uninstallation of the application does not work. 
However, I did not agree with them and required for more steps about uninstalling the specific application and studying the system why the condition exists. 

I also gave 1 star rating for SwiftKey to signal the current problem. I will remove it as soon as possible the problem is understood and fixed in both sides.