You're right, but Highscore Digital is a service provider for everything related to websites (Wordpress), they should actually know what they have to do. 😉 

..the mentioning/reply doesn't work for quotes anymore, I didn't get a notification either

So you don't have to create a new anonymous member first.

Because we are schooling a whole lot of wpForo guys who are admins in label only.

They do not know that you cannot actually change a username in Wordpress unless you edit your DB directly (danger danger). They will probably forget to change the email address of this pseudo anonymous user (the username won't change, only the display name and nickname). They will also forget to delete all subscriptions this user may have.....

Best forum practice since time immemorial is for site admin to create the user called anonymous from scratch with a dummy email address etc. It will also prevent the usual idiots from registering on your forum with the username anonymous.

*P.S. is just me or is mentioning/reply notifications not working on wpForo? Both email and in the widget.

Trying to give them a new name each time such as Tutrix mentioned will result in too many silly names such as anonymous123 etc.

Why, you can add all the posts of future deleted members to this first one.
So you don't have to create a new anonymous member first.

is no possibility to only remove the author's information?

As an admin, you can edit the member's profile and remove all author information.
Give them a new name, such as anonymous

Thank you! I can only see two options; "Delete all wpforo content" or "Attribute all content to xy". So i guess there is no possibility to only remove the author's information?

Is there an option where I can decide what happens to the posts?

Thanks for your help!