Adobe is going to stop releasing and updating Flash in 2020.
This does not mean that every website is going to stop using it. There will be ones that will just happily carry on using it without updates and only change when something catastrophic happens that forces their hand.
They're going to stop distributing it, which means users won't be able to (officially) download it anymore. Most browsers are already actively blocking it to some extent even if you do have it installed. Mobile abandoned Flash years ago. Any public-facing site that requires Flash will absolutely be taking an audience hit for continuing to require it, if they aren't already.