Should I be concerned that shopify/draggable is no longer worked on by Shopify, appears to have only one remaining maintainer,  has been tagged "beta" release since 2017, no commits of substance since mid-2019, and the latest github release (also from months ago) wasn't published to NPM?

These are all huge red flags for "abandoned package" to me.

 ☒ I think that the red flags are warranted, but also it should be easy enough to update the code to swap in a new library.  I've recently played around with dragula and interact.js on a project where I originally had draggable. In preparations of this episode, it was difficult to find a good maintainable library that wasn't reliant on jQuery.

something like jQuery sortable/draggable could have been used instead, and I don't get the reason for using stimulus here... could it have been done without stimulus?

 ☒ yes, you could jQueryUI and StimulusJS isn't an absolute necessity, but it does make things a bit easier with the selectors and keeping the code more organized.