I could be mis-remembering but I think you're right, the option used to exist.
That said, it was a bit of a misnomer if you ask me. From memory, the only options you could choose between were UNII-1 channels 36-48. That covers the 80MHz spectrum of radio that would be used regardless of which of the individual channels you actually selected i.e. you could select 36 or 40 and the hub would broadcast across exactly the same 80MHz range of channels regardless 🤷
The other thing to consider with the Smart Hub Plus is that it can operate at much higher channels (UNII-2/3), some of which can broadcast at higher power and are therefore preferred. If you were to fix to the 36-48 range, then you lose out on the benefit, and also possibly inhibit the ability for the hub to operate at 160MHz channel widths (another thing 'new' to Smart Hub Plus, that pretty much doubles the Wi-Fi capacity for devices that can make use of it).
