Maybe I'm the odd one out, but I feel like the instability is mostly surface-level and FE is not, actually, so much less stable than other areas? I got into tech via web development, migrating further into the backend over time, and about four years ago became pretty much exclusively backend. Just started a new job and I'm back in the front end.
Some things have changed, but not that much.
- Typescript was just getting popular when I left, now it's basically a standard, but 1) after having been in javaland for a while, adding typing to JS makes a lot of sense to me and 2) though it's a change, feels like a change toward more stability, not less?
- React was the big framework when I left, still is the dominant framework.
- There's still a lot of option -- eg vue, etc. nextjs wasn't a thing I knew about that I am starting to learn now. Class syntax felt unecessary to me but seems to be de facto these days. But... these all feel relatively small?
I started in FE back in the days of IE 6. jQuery was a godsend when it arrived, underscore was super helpful. I'll grant the shift from "we use JS to add some interaction, form validation, and sometimes interact with Flash" to "we use JS to write actual programs delivered via your browser" was a huge shift, but since we've made that shift it seems most of the "instability" is more around FE devs adopting practices already common in the rest of the stack.
I think maybe that's where this sense of instability comes from -- a focus on the wide array of possible _implementations_ -- but I feel the really disruptive shift from "interactive decoration" to "web applications" was the really big one, and that's a while back now. And even with the dizzying array of possible implementations, there's obvious market leaders -- you're not going to go wrong by learning React, any more than you're going to go wrong by learning Spring Boot. Always good to explore and get familiar with other libraries and frameworks, but if one feel overwhelmed by options, there's some clear well trodden paths that aren't changing _that_ fast.
One thing I will say is that I do think FE is _harder_ than BE, because the scope is wider. You need to grasp all the aspects of good programming, but _also_ get a decent grounding in design and UX, which are their own disciplines. A backend API doesn't have to think about things like screen readers or color contrast or all the weird ways users will figure out to misunderstand how to interact with your GUI widgets. A UI for humans is inherently harder than a UI for machines. But that's not the same thing as saying that the FE is unstable.