The folder "A.B" in Outlook 2016 with ActiveSync can't be deleted with
Outlook.

That is normal IMAP behaviour, and has nothing to do with SOGo.
You always have to remove subfolders first.
Else the parent folder, its content and the privileges on it are removed.
But the subfolders, their content and priveleges on them are preserved.

Sorry, but I do not agree with you.
Outlook e.g. or Microsoft Web-mail for Exchange allow this and
execute this correct.

If the Sogo Web-mail allows this action, then it must perform it correct.
If the Sogo Web-mail is not able to perform it correctly, it must
give a message, which tells the user to delete first all subfolders
and the Sogo Web-mail should not change then anything.
Nevertheless, in real is the Sogo Web-mail changing something, but not correct.

However, allowing the action and without performing it correctly is really nasty
for the naive user.

I agree with Colmar0. While technically being allowed and correct IMAP-behaviour, you can't expect a user of the web mail application to be an expert in those topics. That's the entire point of having an abstraction like SOGo, otherwise you could just tell people to "telnet" into the host directly.

That's overdramatic of course, but the user interface shouldn't allow "illegal" actions where they can be prevented. That's one of the most important things when designing an interface. The user shouldn't be able to "muck it up" and should not need to care about the underlying technology as much as possible.