Save it in one of the global tables.
Global tables? As far as I can see each site gets its own options table. I cannot see an option table that isn’t associated with a site. I was going to just use the site with id 1 to store global settings (under a different key) so that all sites could just reference that to fetch the global settings. However what if the user deleted the original site? Then there wouldn’t be a site with ID 1 and that would fail.
Found it. get_site_option, update_site_option. On a network site that is global settings.
Also, blog #1 never ever gets deleted. at least not normally.
Not normally but it is possible.
And inadvised. The first blog in the network is the default single WP you started with.
From the backend, there’s no delete button for the main site. You’d have to deliberately rearrange the network to get rid of it.