Some personal thoughts on the PR & Ideas:

1. I like that it improves consistency across the codebase and should help understanding the difference between shallow and "normal" (deep) refs
2. Could it become more confusing for new contributors to look through the codebase as it strays away from the classic Vue naming convention?
3. Besides inconsistencies, this should improve perf (I assume) - but does it actually? Is it quantifiable or more of a micro optimization? I suppose with one (if not the) most used Vue 3 library, this could definitely benefit some applications but I'd be curious when that point is reached/noticable for a few of the changed composables.

1. With the ESLint rule, contributors should get the error with hint to choose between deep and shallow - this brings the awareness of what ref you are using.
2. This PR itself does not improve or change the performance (there should be follow-up PRs that changes some deepRef to shallowRef, which would actually improve the performance, but that would be breaking and we need to handle it carefully). But more like a move that we are trying to make the shallow default. You can argue that most of the time performance is imperceptible even on nested objects/arrays (as long as they are not crazily big). Micro or not, I consider this not an "optimization", but more like a default performant practice that should be the default, where optimization is not even relevant.

Thanks for the insights @antfu!

Do you think the move to shallowRef by default (similar to how other frameworks do it) would be one that should be adopted by the whole ecosystem, so also for "end users"? If yes, why that? 🤔

I don't honestly sure what would be the best practice for this. Theoretically, yes, I think ref should be shallow by default and ideally the ecosystem should move on that, but the status quote has a strong inertia, where benefits might not always justify the cost of migration. I think we still need time to try and feel about it, there is no final answer yet. But I think VueUse could a good place to start and explore.