Why should we care about Adobe Flash?

The short answer is that it takes a loooooong time for software to die. Even in 2018 we still have COBOL running multi-billion dollar companies, despite COBOL being a "dead" language for decades.

The longer answer is there's still a significant amount of websites that require Flash, and people re-enable Flash for practical reasons.

Oftentimes these are "mission critical" internal corporate websites or schools that haven't put a priority on replacing legacy applications based on Flash. This might mean using older browsers where Flash isn't disabled, or just users being trained to re-enabled it every time.

Across the board, the numbers as of April 2018 are around 5% of websites according to https://w3techs.com/technologies/details/cp-flash/all/all

So I wouldn't say Flash is "dead", but it is slowly dying.

Because it's not completely "dead". It's just suppressed, for example, in Chrome the user has to click to allow Flash.

Google has said that by 2020 it will not support Flash at all.

Unfortunately, a lot of corporate software or internal websites still require Flash for various things (and not necessarily a recent version that may have some patches). If a company decides that their internal application requires a five-year-old version of Flash to simply work, they're not going to patch it.

That leaves an awful lot of software and sites that are likely vulnerable to any new attacks based on Flash.