just tested the functionality with the Alexa V3 Skill and how Openhab items are recognized in the Alexa app.
I just created a sensor item in OH as a Contact and started the Exploring of new devices with Alexa. Alexa did find the new device and it was recognized as an contact, but I wasnt able to use the device in the Alexa app to trigger a new routine. It just says that I havent devices that could be used for that.
If I create a socket on OH and integrate it in Alexa, I can use it in routines as an action. Maybe thats a bug in the new Alexa V3 skill.
This is actually a known issue. It is actually mentioned in the contact and motion sensors documentation. Proactive reporting is required for that support but not yet implemented in the skill. Basically, currently Alexa-linked OH item states are only updated via polling requests opposed to pushing the updated states directly from the OH server. This is part of the asynchronous enhancement which will be a major change.