The YC has "Foot Controller" ports, but I cannot find an assignment parameter to solve for this issue/need.

Don't have access to a YC but the Owner Manual, on page 50, appears to say you can select what the controller affects by setting the Receive SW parameter:

Determines whether each Voice Section receives (On) or ignores (Off) the
corresponding MIDI messages received from external devices or the messages
generated by operating the footswitch or foot controller

You might try that.
https://usa.yamaha.com/files/download/other_assets/5/1376845/yc88_yc73_en_om_a0.pdf

how could this "need" be presented to Yamaha for future IOS update

You can submit suggestions on Yamaha's IdeaScale page or use the 'Contact Us' link at the bottom right of this page.
https://www.yamahasynth.com/learn/news/yamahasynth-ideascale-community-faq
.

You can choose what the Sustain pedal is set to do on a per Live Set location.
These are referred to as the “Settings List” — means the list of your preference for this particular Live Set Sound. Each program can have a different configuration.
Those items referred to as the “Menu List” are global settings for the whole system.

So on a per Live Set Sound location basis you can choose which section “Key A” or “Key B”, both or neither has their “Receive Switch” = On for Sustain. The default is both sections (Key A and Key B) are set to On.

Take your time. Remember - you never really finish reading a Manual. Use it when you have specific questions. There are plenty of articles and videos on these basics right here on this site… go to the “LEARN” section.