The setup wizard says it, clearly, the very first time anyone reaches it: this name won’t last, choose your own. And then the wizard finishes, and it never says anything again.
A warning that only exists at the exact moment it’s easiest to ignore
The trouble with a one-time message is the timing of the one time. It arrives while someone is still exploring — clicking through screens to see what the thing even looks like, not yet in the frame of mind to make a permanent decision about a domain name. Click past it, look around, come back tomorrow, and the warning is gone. Not resolved — just no longer being said. The server is still wearing a borrowed name. Nobody’s telling it that anymore.
Borrowing the shape of a warning that already works
There was already a banner that does exactly this job, for a different concern — a strip of colour at the top of the page that shows up for as long as something needs attention and disappears the moment it doesn’t, with no memory of having been dismissed because it was never dismissible in the first place. Re-checked on every single page load, honestly, every time.
The new one asks the same question the old one always has, just about a different fact: is this still true right now? If the server is still answering to its own placeholder name, say so. The instant it isn’t — the instant someone actually sets a real one — the banner has nothing left to say, and says nothing.
What actually changed
The one-time nudge during setup still exists, unchanged. Sitting beside it now is something that doesn’t go quiet just because the moment for it has passed — a plain, honest strip at the top of every page an admin sees, present for exactly as long as the thing it’s pointing at is still true, and not one page load longer.