I've updated from 6.10.10 to 7.17.2. Every time a user loads the homepage I'm getting two times the warning message "DEPRECATED: mxui.widget.declare will be removed in version 8.0. Use dojo/declare instead." in production that would be a lot of messages. So I'm trying to fix this before going all the way with production environment. Found out that there 20 widgets in the project. I've removed 8 from the project. The other 12 I've removed one by one, fixed all the errors then run the application to see if the warming is still there. If it's still there, I've restored the one and moved on the the next one. Done that 12 times with no succes, no idea which one is causing the warming message.
How do I fix this message?
Did you eventually manage to solve the warning message? I'm in the same situation as you described, also I've already created a support ticket for Mendix.
You can search for the text " mxui.widget.declare" in the folder deployment\web\widgets for sources with a tool like agent ransack.