Is it true that Custodian brokerage accounts have a complete loss risk?

Hmm no I do not think that is true.

At least according to Standard Chartered... "Should the Bank decide to cease the offering of the Online Trading platform, the Bank will, at every client’s option, deliver the Documents of Title of the securities in its custody back to the client, or transfer them to a bank or agent of client’s choice."

You can take a look at SCB's FAQ here: https://www.sc.com/sg/help/faqs/bank-with-us-faqs/, which is where I found that information from.​​​