In practice, people - newcomers to haskell in particular - will be using older versions of stack approximately forever, just like with cabal-install or any tool. Better if we can avoid breaking the new user experience in this way.
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I’m going to presume here that one of the goals of migration is to be completely independent of the “previous system”, just in case it disappeared forever.
So (hypothetically)…if “something happened” to FP Complete, the old URL would be broken permanently: there also goes the “new user experience”. To me, the “middle option” is to keep the old URL around until support of the final pre-migration version of Stack ends: then the old URL can be deleted.
Now if at that time there are still people using pre-migration versions of Stack, then they’re are either very overdue for an upgrade anyway, or they will have to learn how their version of Stack works so they can support it themselves (in much the same way someone using an old version of HBC, NHC, JHC, Hugs, or even Yale Haskell would have to). I doubt very few of them would still be new Haskell users…
I am relatively relaxed about this because I follow the places where many Stack users congregate. In practice, the number of queries that arise because somebody is using an out-of-date version of Stack are relatively infrequent and, in the case of this URL change in particular, rare (so far).
There are a number of places where Stack users are prompted to consider whether or not they are using the latest version of Stack, including in issue templates at Stack’s repository and by the Stack executable itself: Configuration (project and global) - The Haskell Tool Stack. GHCup users are given recommendations if they use ghcup list or GHCup’s text user interface (TUI).
The version history of Stack is readily available online - Version history - The Haskell Tool Stack - and that gives users a visual impression of how out-of-date is their current version of Stack.
Yeah so looks like https://s3.amazonaws.com/haddock.stackage.org/ is indeed finally gone.
So presumably the transition period ended. ;o)
I was optimistically hoping it would stay around much longer:
I am going to scamper now to update Fedora stack’s url.
Yeah there’s nothing we can do about amazonaws.com urls since they’re not under our control. 
The bucket has been deleted now, so people will simply have to update the url (or upgrade stack).
Furthermore, haddock.stackage.org/snapshots.json is now also unreachable. Although that url is under our control, it seems we’re unable to point it to the right place using DNS alone. I can’t make haddock.stackage.org a CNAME for stackage-haddock.haskell.org due to Cloudflare’s security policy (one can’t make a CNAME across CF accounts, which is what would happen here). And the attempt to rewrite the specific URL haddock.stackage.org/snapshots.json to stackage.org/download/snapshots.json seems to have no effect.
There are other things I could try but I don’t have the time right now.