There is a custom setting you can configure:
EnableKeepAlive Defines whether the web client sends a keep alive request every SessionTimeout/2 milliseconds, to prevent a session timeout. default value true
I assume if you set this to false, the client will eventually time out, perhaps based on web server settings? I have not tested this.
Edit I do not currently use the Mendix cloud, but according to the docs...
All configuration that is specific to the application environment you're logged in to is contained in a configuration file, that is named m2ee.yaml, located in the .m2ee folder in the home directory of the server account. When your user account is example, this means the location of the file on the server file system is probably /home/example/.m2ee/m2ee.yaml. This file is a text file in YAML format, and has a fair amount of inline documentation.
See this link for more information
No, but I would prefer something like this. Could you file a support request for this? There is also no way to know when a user is inactive. Only by tracking this yourself you could track those users and log them out. But preferably it would be configurable in the platform, like you mentioned.