Keybase is a good example of the centralized “secure ledger” described in the article you linked:
> Keybase's approach to server security: (1) each user has his or her own signature chain that grows monotonically with each announcement; (2) the server maintains a global Merkle Tree that covers all signature chains; and (3) the server signs and publishes the root of the Merkle Tree with every new user signature. This configuration strongly discourages the server from lying by omission, since clients have the tools to catch the server in the act.
Source: https://keybase.io/docs/server_security/merkle_root_in_bitco...
The post by Beaker's creator, linked in this article, is also worth a look: https://pfrazee.github.io/blog/secure-ledgers-dont-require-p....