Multisite networks

Activate it on one site or across the whole network. Each site keeps its own settings, its own subscriber list and its own counts, so one site being closed says nothing about any other.

Network activation sets up every existing site, and a site added afterwards is set up on its own the moment it is created. A site deleted from the network takes its subscriber list and its counts with it, and its page for WordPress updates. A site deleted while the plugin was switched on for that site alone leaves its table behind at the time, and deleting the plugin clears those leftovers away. Deleting the plugin only removes data from the sites that asked for it, because the opt-in is a per-site setting.