Finally found a reason where Use Year is relevant to me!

[eaglesrest]

The full warning message is "Ineligible Date Span
The dates you selected span 2 Use Years and can’t be booked online.

Please adjust your dates to start on or end before Mar 1, 2019.

If you want to proceed with the dates you selected spanning 2 Use Years, please contact Member Services at (800) 800-9800 to assist you. You must be 18 years of age or older to call."

Maybe worth giving them a call to check?

You could also try doing it via an online modification though I think it's a long shot. If you have points in this UY and next, you could try a dummy booking end November for 1 or 2 nights. Then try and modify it to move into December UY fully. Obviously don't commit to banking or anything but just see what errors pop up. I'd do it but I've already banked into next year.

[YorkshireT]

I thought walking would become much more prominent.
This sounds like an unintended consequence where the system hasn't been programmed with this in mind.