It is not possible to give the reason for offboarding as it may tip off fraudsters on how they can evade detection next time. This is standard across the industry.

i.e. "Security through obscurity" - which isn't a phrase with a good reputation in these parts...

I note that if you know your way around the dark web, it's relatively straightforward to find a "financial crimes advocacy" website with suggestions on evading KYC and other compliance hurdles; obviously I won't repeat them here (and I know that probably the majority of would-be bank fraudsters probably aren't into checking the darkweb for advice) so I hope there's more to it than just information-control...

It's a great point, but I'm afraid this is what the regulators enforce onto the banks/fintechs, out of our hands!