As a follow up to my question,
This is the Answer I received from SQUARESPACE Customer Care ( ten days later, on Feb 24,2021 ) regarding the STRIPE's API Upgrade:
" That Stripe API upgrade wouldn't impact our integration with Stripe, so that wouldn't be directly linked to the way we integrate with Stripe. We would only recommend upgrading the Stripe API version if you know what is that for, since even though this does not affect our integration with Stripe, it may affect any custom tool you may have created to access that Stripe's API. "
So, it sounds like it would be OK to accept the Stripe API upgrade. In theory, all NEW Squarespace-Stripe commerce customers, today, would be signing up with the most current STRIPE API in effect.
I will update this post in a few months with my experience ( positive/negative) - just to be complete.