It would be nice to be notified if one is removed from a group e.g. via mail.
Why do you think that? I think, it should be optional, if at all.
Because without a notification, users have to check their groups regulary to find out whether or not they are still members. A notification would help to quickly find out directly after one has been removed by a group admin.
Making it optional seems a bit too much, at least to me. However, if there is a scenario in which a notification mail would be perceived negatively, it might make sense to allow the group admins to opt out of sending the mails.
That's what I meant with optional.
Regardless, I would try to mimic the behaviour of a well-known social network for this. There is only a notification if you are invited or accepted to a group, not when you are thrown out of a group. So I do not think that this is an important enhancement, if at all.
I agree with Stephan and we should notify users but have an option to switch it off.
Actually, in cases of using the typo3-client for the publication index, e.g. ITeG, not knowing you've been removed from a group would be fatal.