You could use these shell script versions of pipelines in GHA though, right? There is nothing stopping you from triggering a bash script via a "run" step in YAML.

These reusable actions are nothing but a convenience feature. This discussion isn't much different than any other supply chain, dependency, or packaging system vulnerability such as NPM, etc.

One slight disclaimer here is the ability of someone to run their own updated copy of an action when making a PR. Which could be used to exfil secrets. This one is NOT related to being dependent on unverified actions though.

(re-reading this came across as more harsh than I intended.. my bad on that. But am I missing something or is this the same issue that every open-source user-submitted package repository runs in to?)

I'm trying out SelfCI [1] for one of my projects and it's similar to what you were describing. My whole CI pipeline is just a shell script that runs the actual build and test commands, I can write a script in another language like python if I need more complexity and I can run it all locally at any time to debug.

[1] https://app.radicle.xyz/nodes/radicle.dpc.pw/rad%3Az2tDzYbAX...