User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-31040770-20201205102259/@comment-1038387-20201206094129

It would still be tied to the user accounts so the the unsupported software would still have to be patched, otherwise they've got a huge security hazard.

As for the redirects, that's unrelated to UCP. The redirects were not updated on the domain move (though they can be on request). However, new wikis can not be created with the fandom.com version of the redirects. So "food.wikia.com" redirects to "recipes.fandom.com", but "food.fandom.com" doesn't (or didn't) I've pointed out this glaring issues shortly after the domain migration. It's especially annoying with the language-wikis screen popping up if there's no en domain, but there are foreign language versions. Say, hypothetically, the English wiki is at mylittlepony but the Spanish one is at mlp, and mlp.fandom.com was not a redirect... then it actually says there's no mlp wiki and you can create one (but you can't). But the language-wikis screen is another can of worms entirely.