This is a rather niche plugin, but in case anybody (any IndieWeb enthusiasts around?) has a use for it: I adapted Selfauth to run as an integrated Kirby plugin.
Selfauth is a self-hosted Authorization Endpoint used to login with a personal URL (as Web sign-in) via IndieAuth.
So, my plugin is essentially a wrapper to integrate Selfauth with the Kirby backend …this way, it is possible to log in to IndieAuth-enabled websites by logging in to the Panel. The result is the same as following the RelMeAuth cookbook recipe, but all self-hosted, without third party involvement.
https://github.com/sebastiangreger/kirby3-selfauth
Some more info on my blog and in the readme.
I’ve been using it myself in production for a few weeks and have tested it with a fresh Starterkit. Use at your own risk, but please let me know if you encounter any issues, need help or have other feedback 
I just tweeted about it and also added it to the plugin directory.