Adobe
I keep getting a window popping up telling me to uninstall Adobe since it will not be supported after 31 December.
Is this correct and, if so, what will replace Adobe?
iMac 27″ 5K
That is correct, it has been replaced with HTML5 and you should no longer need or use Adobe Flash Player.
see > https://www.adobe.com/products/flashplayer/end-of-life.html
FWIW I haven't had Flash Player on any of my Mac's for over 2 years and have not missed it one bit. This is nothing new and most websites are way ahead of the game and have already moved away from using Flash Player.