It's been suggested in the past. The general outcome of the conversation was that it's not worth writing because if a users nick is about to expire or has expired then they aren't interested in IRC or your network. I for one agree.
For this to be implemented would take a considerable amount of cpu cycles because the same check used to expire the nicks would need to then also check for near expiration thus doubling the workload, then take even more by sending emails to people that are most likely uninterested in the first place. On very small nets, this feature would go relatively unnoticed, however on larger nets could be a catastrophe. To the smaller admin I say, lengthen your expiry.
Another thing you can do is keep in touch with your users with a regular newsletter outside IRC so users don't "forget". It not only makes the user feel more important but makes them feel like they are apart of your community.
