Shumway is not under active development. The actionscript runtime itself is pretty good, but getting compatibility with the Flash API surface turned out to be practically unachievable.
The only real* browser I can think of that isn't listed there is Opera, and they're only barely hanging on. The numbers I've seen peg them at roughly 1% of desktop browsers, mostly in Europe.
*: That is, one that isn't just a reskin of another browser on the list.
Of course that describes what they are doing now, not their plans. Which is not surprising, because Apple never says anything about plans, as a matter of corporate policy...
Whoops, I totally didn't notice that was missing from the list.
That being said, while Apple hasn't explicitly said they plan to remove Flash from desktop Safari, they've implicitly signalled it -- they've already got features in place which make it often only load on demand.
There's not much to remove. You install Flash on macOS separately and Safari will load that in as a plugin. It seems obvious that Safari will remove that plugin functionality on or before 2020.
Safari never shipped with Flash to begin with, and Apple has regularly published metadata updates, which blacklisted old, insecure versions of flash that the user might have had installed.
