> would be groups of banks or local regulators trying to define common standards

Why 'would be' just out of interest?

AFAICT Open Banking is an organisation that has been given a mandate by the UK government, through the competition and marketing authority, and is funded by the nine largest retail banks. In the UK it is the defacto standard, and compliance of the CMA 9 is mandatory.

While there is so far no consistent standard across the EU, at least within the UK this one is set and pretty much non-negotiable.

(Disclaimer - I have consulted with Open Banking and continue to do so, but of course I do not speak on their behalf)

-- edit --

I'm particularly interested in this -

> Third-Party Providers or Aggregators (Plaid, Teller, Tink, SaltEdge, Bud...) - would essentially provide access to the accounts of multiple banks via APIs.

As AFAICT this would be explicitly disallowed unless all the users of said APIs are themselves accredited. You can't just get accredited for PSD2/OB API use, then expose that information to non-accredited entities. If this is what Plaid are doing then I wouldn't expect their accreditation to last all that long.