Contributor Guide
Iridis is meant to be as forward compatible as reasonably possible, don't try to over-abstract everything so that the api becomes limiting and incomplete, but avoid using blank vanilla code / auto-abstraction for things who's structure may change frequently. Also if you don't like the way mojang provides an api then that's another valid reason for abstraction.
How to write Mixins for Iridis (this isn't a mixin tutorial)
Making Events for Iridis
In theory updating shouldn't be that hard, since fabric updates to snapshots we can get a head start
A simplification of the bridge api is essentially, we use the new api to implement the old api rather than using the game. This way mods built on the old api will continue working, but we can still remove legacy code and clean the api. In theory we could run this forever, and mods would never break, in practice this is probably also possible, considering bukkit exists.