Safari is the last non-evergreen browser. As long as Apple insists on this policy it doesn't matter how good a job the WebKit team are doing; they're still hamstrung by the outdated release policy Apple enforces.
Luckily, soon Apple will be forced to allow alternative browsers. At that point, I hope Apple will spend some of its trillions on actually improving Safari, in order for it to stay relevant and competitive.
Safari seems to be the last bulwark against a Chrome-only web driven by features of the week.
Full Chrome/Chromium on iOS could very well end that, and I'm not thrilled at the prospect.
Personally I appreciate Safari's resistance to constant featurism in general, and conservative approach to features that are likely to compromise power usage or privacy. I have little desire for web apps to reach complete feature parity with native apps, or to have complete access to all hardware and OS features. Many developers may hate it and want to usher us into Google's PWA future, but I'm absolutely fine where things are right now. And I certainly don't want to have to use Chrome/Chromium for everything.
Lastly, as a developer, I don't like building on quicksand. And yet that's often what it feels like using web/JavaScript frameworks and browsers that change from week to week.
Nonetheless, OP's recommendation that Apple note which versions of Safari a certain bug is fixed in seems like a very good idea that should be adopted.
The pace of change in the frameworks world is completely independent of web and browser features. And the web is very backwards compatible — so new web platform features don’t rot the foundations of any existing web apps, assuming they use specified features.
Just pause for a moment and think who's proposing these standards. The same groups pushing for browser "featurism" sit on the committees that define these "standards". It's not meaningful to say that they're different things when they're all part of the same effort in different disguises.