Like what exactly? I played/viewed a lot of Flash content back in the day, but I don't remember anything out of the ordinary that simply isn't possible/is extremely difficult today, want to jog my memory?

There was a period of time after Flash was deprecated and web technologies weren't ready to replace it, but I think we're beyond that today, unless I'm missing something essential Flash could do.

Instead of citing a particular technology (whether Flash, HTML5, WebGPU, etc), which risks getting into the weeds of defending any shortcomings in one or the other, I'd rather propose that client-side rendering in general is still under-utilized and capable of so much more. I also think the under-appreciated elephant in the room is that Apple and Google have both been guilty of subtly manipulating web standards processes to nerf client-side applications. And they've been very clever about it. Sometimes proposing potentially exciting enhancements but later sabotaging them - always with some plausibly deniable cover, whether security, backward compatibility, accessibility, etc. Other times they'll 'embrace and extend' a new proposal - and then keep extending and extending until it either becomes practically unimplementable, unperformant, bloated or just collapses in committee under it's own gravity.

Bottom line: powerful, performant client-side applications securely delivered in real-time through the open web are bad for app store walled-garden models and businesses that rely on controlling centralized social media or gaming platforms. Advanced client-side technologies and standards aren't as good or widely deployed as they could be because powerful interests don't want them to be.