Question How do I Know, that the user has opted out?

Yes there is,

This method calls the opt out function, an ideal use case, would be to have a button which calls the function that the user can then press if they don't want to have any data collected on them.
You can then use a bool and save it then do a check at start up.

Code (CSharp):

1. public void OptOut()
2.     {
3.         try
4.         {
5.             if (!consentHasBeenChecked)
6.             {
7.                 // Show a GDPR/COPPA/other opt-out consent flow
8.                 // If a user opts out
9.                 AnalyticsService.Instance.OptOut();
10.              }
11.              // Record that we have checked a user's consent, so we don't repeat the flow unnecessarily.
12.              // In a real game, use PlayerPrefs or an equivalent to persist this state between sessions
13.             consentHasBeenChecked = true;
14.         }
15.         catch (ConsentCheckException e)
16.         {
17.             // Handle the exception by checking e.Reason
18.         }
19.   }

You kind out more information in the documentation here: Data privacy and consent (unity.com)

 

Yes, this was for Legacy Analytics.

If the player wishes to opt-back in,

Opt-BackIn#
Unfortunately there is no easy method to opt back in.

There are three options:

1. Delete PlayerPrefs and restart application.
2. Clear all data and cache for application.
3. Uninstall application and reinstall.