Well, I know for a fact that all of the banks in UK must support open banking. Granted, the functionality is mostly reserved for B2B communications, so in practice you will need to go through a OB provider. These providers [try to] abstract away the differences between individual banks and provide coherent APIs to their customers.

(I deliberately avoid the word "stable" because everything in this space is still in flux.)

Semantically OB is a bit like OAuth. The end user authorises an OB provider to conduct [some] operations on their bank account, and the OB provider exposes their chosen functionality to the user. Due to the unit economics[ß] it will likely be quite some time before OB for retail customers can take flight.

I would have personally expected that the first uses of OB for retail customers would have been personal accounting services, because those could do with read-only authorisations. But the challenger banks exposed this natively in their apps, adding sufficient friction for third-party services to take hold, and in fact made it such an attractive feature that legacy banks have had to race to implement similar functionality.

ß: Businesses do a lot more transactions than individuals, so the overhead per retail customer remains high.