A user could have multiple devices using the Alexa app. Each app has it's own Oauth2 session while still for the same user. The Google API sends the access token with the disable request. For Alexa skill event it's is the user ID. It is not possible to determine which session on which device the skill is being disabled. It would be preferable if there would be an smart home event with the access token to tell which session is ended, so that is can be handled by the same 3rd party API endpoint or lambda that handles all other Alexa requests.
Or... will disabling the skill on one device also disable all skills on all other devices for the same user ?