Whatsapp is disappeared in Ultra power saving mode
You can use what's app just go to gallery select any photo to share and then you will get 3 options 1 one is whats app just go ahead and select any contact when whats app is about to send the photo click on upper left hand corner what's app back button ...

Then you will able to use what's app without any problem
Just remember do not to clear your recent apps if u clear that part then you have to do the same processes again