As someone who did a bunch of work on Ruffle a while back, "mercy kill" is
almost the correct word. There's about a decade it spent rotting before the actual kill, and Apple's not the one who fired the final shot. I've heard stories from both the Apple and Adobe side on this, but basically both companies wanted Flash on iOS and neither of them were capable of actually shipping a good version of it.
Apple begged Adobe to ship a working Flash mobile build at least four times and each time they rejected it for all sorts of various UX or performance issues. At one point Apple asked for and was delivered Flash Player source code, which they reportedly couldn't get to compile. Adobe tried to brand Flash as an open standard, and then went over Apple's head by just shipping an AIR runtime that could be packaged into an IPA and submitted to Apple. Jobs then wrote the infamous "Thoughts on Flash" letter, which was really there to justify going scorched-earth and banning all third-party development tools. This only lasted for about three months before the Obama DOJ threatened to sue[0].
Also, Steve Jobs was probably pissed off that he couldn't get the CEO of Adobe on speed-dial. At that point in time everyone involved with shipping iPhone software was in his contacts and in regular contact with him. Google logo looks weird on the phone screen? Have Jobs call Page and get it fixed in 10 minutes.
As it stood after that moment, Flash was a viable development platform for iPhone apps and remained so for many years. This is entirely separate from its use in the browser. Practically speaking, you have probably played plenty of Flash games on iOS without even knowing it, because all the hard work of building touch-friendly UX and a performant UI was shunted over to the developers of individual games rather than trying to make, say, the core Flash rendering model GPU capable[1].
Adobe then shipped Flash Player for Android to huge fanfare, and it sucked just as hard as it did on Apple's dev iPhones and was unceremoniously canned a year later.
At this point it was obvious Flash Player needed a rewrite, even within Adobe, so they announced "FP Next" along with an AS4 language for new movies to run in. Except the Adobe execs were angry about the cost so they tried to shake down their customers for the funds. They wanted any cross-compiled 3D engine code to have to pay a revshare to Adobe. Everyone jumped ship to Unity, so Adobe canned the revshare requirement... and FP Next/AS4, the thing that was supposed to modernize Flash's aging codebase.
And then right after Adobe starts disinvesting from Flash, a bunch of CVEs land and all the browser vendors pushed hard to actually, once and for all, excise plugins from the browser. That was the actual mercy kill, but it was preceded by almost a decade in which all the people who knew how Flash actually worked didn't have the budget to fix it, and all the people who wanted it fixed didn't have the expertise to do it.
[0] For the record, Obama was the guy who saw Zuckerberg illegally buying Instagram to keep people from moving off of Facebook and said "sure thing, wave it through".
[1] There's an AS3 project called Starling that gives you hardware rendering by pre-rendering a bunch of assets in advance into bitmaps, which kind of betrays the whole point of Flash. But I also can't imagine Adobe doing it any other way as the Flash renderer was both highly optimized and bespoke.