You could link to an off-site webpage before the ban, and then post details of how to connect to the new site after. And it's not like users couldn't have had

You could link to an off-site webpage before the ban, and then post details of how to connect to the new site after. And it's not like users couldn't have had contacts between each other and the moderators and couldn't spread the word about the new community. That's not to mention the ability to evade bans to spread the word on Reddit itself, if need be. Spreading the word is actually the relatively easy part... it's migrating all the old content to the new site that isn't made easy by Reddit, because (afaik) it doesn't offer any kind of export feature.. but the site could be scraped.