I'm currently writing a library that implements SimpleFIN. While the main website hasn't been updated in a while (even the demo endpoints are out of date, and I've written to the maintainer last month about it[1]), the demo endpoint described on the Simplefin Bridge the docs still works[2] so hopefully there's still movement.

[1] https://github.com/simplefin/simplefin.github.com/issues/14

[2] https://beta-bridge.simplefin.org/info/developers