I am using Mac OS Sierra 10.12.5 (16F73) and Safari 10.1.1 (12603.2.4).
When I visit youtube most videos play fine. but some videos say
Flash out-of-date
It also quickly bring up a prompt to say "upgrade flash" which leads me to Adobe website.
But wasn't flash totally discontinued on Mac and Safari? if so, why am I being asked to upgrade flash?
I don't want Flash on my machine.
My understanding was that Youtube/Safari should use everything as HTML5 (H264) stuff. Then what's going on?
Flash is still [barely] alive & kicking, though is not needed at all for YouTube these days.
The best way to ensure it's gone once & for all is to use the dedicated uninstaller, available from Adobe, at Adobe KB: Uninstall Flash Player | Mac OS
The uninstaller is Mac version-dependant, but the direct link for OS X 10.6 & later is at https://fpdownload.macromedia.com/get/flashplayer/current/support/uninstall_flash_player_osx.dmg
Uninstalling per @Tetsujin's answer is the correct course of action if you want Flash gone completely.
If you don't want it gone, but don't want most websites to know you have it installed at all, check your Safari preferences, Security tab, Plug-In Settings.
Select Adobe Flash Player in the sidebar, then make sure the setting is "Off" for everything except websites where you specifically want to use Flash.
If you set it to "Ask" rather than "Off", the website can still tell that you have Flash installed, and may still try to load the Flash version of its video player etc. before Safari can ask you whether you want to use it. This can lead to the "update Flash" message that you saw, if your version of Flash is out of date.
I found out the answer. it was my own fault. I was trying to install the youtube ad blocker, but by mistake i installed the youtube flash player plugin in safari.
because of that plugin I started getting this warning.
once I removed that plugin, the flash warning went away.
