whatsapp stopped supporting old devices and versions ,so if you have such device ,whatsapp will not work after you uninstall it
read detailed infos here : WhatsApp Blog
That statement was from whatsapp 9 months ago mate.
Times are moving on.
BlackBerry OS and BlackBerry 10
Nokia S40
Nokia Symbian S60
Android 2.1 and Android 2.2
Windows Phone 7
iPhone 3GS/iOS 6
Not many I know nowadays have them.
It wasn't to long ago blackberry was the main smart phone. I've no doubt you will see more of that as its a crowded market and developers don't want to support a phone that's 7 years old.