There is merit in the goal of this, but not at all in the execution.
There are two user experiences to consider.
- User sending an "@message" to a suspended user.
- Suspended user receiving an "@message"
1. Sending a message to suspended user.
All communications on stack overflow are asynchronous.
Like email, you send the message and forget about it. For this reason, sending a message to a suspended user is identical to sending a message to a user who is offline: you may get a response eventually. We wouldn't want the user to send their messages differently to a suspended user, because it would almost certainly make them less charitable.
2. Suspended user receiving a message.
This is a little more frustrating an experience. The suspended user cannot respond, and cannot mark the message as something to remember. The only way for them to keep track of the things they want to say is to manually keep track of it.
However, suspension are not meant to relaxing enjoyable experiences. The 'ease-of-use' while suspended is supposed to approach zero.
Suspensions should only inconvenience the suspendee. This means that the other users should be completely unaware. Any pop-up or notification serves to annoy the other users, and shame suspended user. Both of those are negative side-effects of this feature.