You can upgrade (you may have to pay for a half price upgrade depending on which version you paid for before) without a subscription. It's only if you need a more "extensive download capability" as you mention.
I'm not sure it REQUIRES Big Sur, (I'm running Big Sur), but MAC operating systems do break software so I'll leave it to someone else to answer.
Nothing was updated for USAA in the original OFX area, as MD was using the MD+ concept as their solution for USAA, and depending on Stuart's fix (my documentation) for the OFX, non MD+ solution.
That said, it appears that Schwab discontinued support on Oct 1 for the proxy server Sean set up as a fix late last year, and (it's too early to tell for sure) USAA discontinued support for non-Quicken users (i.e. us) on Oct 10, even with the USAA "fix".
So, maybe MD did the right thing creating MD+ vs fixing specific banks (like USAA) as OFX auto-download support seems to be dropping like flies. Unfortunately (for me), it probably means I will have to get the MD+ subscription at $2/month. Before Oct 1, with both Schwab and USAA working for me, I hadn't planned to do so.
Subscription is only required if you wish to use Moneydance+
Moneydance runs quite happily on Windows and Linux ... :) But in all seriousness I have no idea where the idea that Big Sur is the minimum macOS version comes from. With Big Sur you require Moneydance 2021 as a minimum so there is a dependency the other way round. Moneydance 2022 should run on all recent releases of macOS.
There are no specific changes for USAA, however based on responses from USAA management it does not look like they would be effective. USAA are indicating that their OFX Direct Connect system will go away and companies like Moneydance will not be allowed access to the replacement system, the message from them is use an aggregator and in the case of Moneydance that will be Moneydance+