There are a couple I have in mind. Like many techies, I am a huge fan of RSS for content distribution and XMPP for federated communication.
The really niche one I like is S-expressions as a data format and configuration in place of json, yaml, toml, etc.
I am a big fan of Plaintext formats, although I wish markdown had a few more features like tables.
I like the concept and I think the use case is almost covered by generating API client through generated OpenAPI spec.
It’s needs a bit of setup but a client library can be built whenever a backend server is built.