I worked on (our product) > Backend, and so whenever the frontend was sending me problematic requests I'd file bugs against the "Backend (of the frontend)". Their UI logic was irrelevant to me, more or less; I only observed their "backend" code's behavior.
Of course when I say I worked on the "backend" I mean the part of this product that talked to other backends and storage layers. In a large enough distributed system, "frontend" and "backend" are relative rather than absolute terms...
Then we had to use javascript to do some frontend stuff.
The javascript became frameworks which templated some of the patterns to make the frontend dev easier (led by React and Angular)
Frameworks like Blazor and Phoenix/Elixir are bringing the frontend dev back to serverside.
Even if the server is nearby, there's still many things that can add latency, most of which are not controllable (e.g. wifi interference, bad routers, bad ISPs, train tunnels).
So in general you cannot move all functions to the server because it will be terrible for UX. e.g. Google Stadia.
If you moved to full server-side tomorrow and used Phoenix/Hotwire/HTMX/whatever, these skills would still be invaluable.
With few exceptions, backend/infrastructure roles will still often involve exposing and designing APIs, CLI utilities, etc
I tend to agree, though. Most times when I talk about frontend vs backend profiles, this is what I actually mean.