I worry that CI just got overcomplicated by default when providers started rocking up with templated YAML and various abstractions over it to add dynamic behaviour, dependencies, and so on.
Perhaps mixing the CI with the CD made that worse because usually deployment and delivery has complexities of its own. Back in the day you'd probably use Jenkins for the delivery piece, and the E2E nightlies, and use something more lightweight for running your tests and linters.
For that part I feel like all you need, really, is to be able to run a suite of well structured shell scripts. Maybe if you're in git you follow its hooks convention to execute scripts in a directory named after the repo event or something. Forget about creating reusable 'actions' which depend on running untrusted code.
Provide some baked in utilities to help with reporting status, caching, saving junit files and what have you.
The only thing that remains is setting up a base image with all your tooling in it. Docker does that, and is probably the only bit where you'd have to accept relying on untrusted third parties, unless you can scan them and store your own cached version of it.
I make it sound simpler than it is but for some reason we accepted distributed YAML-based balls of mud for the system that is critical to deploying our code, that has unsupervised access to almost everything. And people are now hooking AI agents into it.