Unfortunately, according to the Google 'Product Expert' here,
there doesn't seem to be any way to disable the notification.
Considering the amount of complaints about it, however,
here's hoping they'll release an update soon
that will allow turning it off permanently.
That said, this news might cheer you up a little bit:
"Apparently the Chrome v78 update will snooze the pop-up for 14 days."
But I believe that every 14 days for a year isn't that great either!
Personally, I have turned Flash off,
and enable it only when prompted by a trusted website –
which, fortunately, happens less and less nowadays.