Honestly no unban all. There is honestly a good reason behind why someone is permanently banned. If there wasn't they wouldn't be banned, or they'd appeal.
When I was staff you could see accounts which had already been banned for advertising(bots) retrying to connect every 5 minutes. If you allow for some of those networks to he unbanned you'd be undoing all the work the members and staff team did to have them accounts banned.
Aswell it already being highlighted that a large sum of toxic people would join back who were banned for that offence. They don't care about whether the server improves they just enjoy ruining other people's experience . If they want to return to the server they should just appeal.
Another reason is a little for selfish for the server. If you release a new version which is successful some players which are banned and whom enjoyed the server before they may purchase an unban.