There are a few reasons why a thread is not accessible:
- it has been deleted.
- it has been moved to or is located in a forum that the usergroup, the user is in, can not access.
- the user needs to validate their email account.
Similarly, if a user gets a no permission screen, when trying to access functionality that has been turned off for the usergroup, then its very useful for the user to understand that. Else they will possibly think this is an error and that the forum software is buggy.
If the user is informed what is going on then they will not need to ask.