Hi
I don't know what device you are updating, but any manufacturer whose update removed/prevented the reinstall of Facebook and Whatsapp would be committing suicide (a few authoritarian governments excepted of course). Even if an update removed these from the preinstalled system apps, you should be able to reinstall them.
As always before a major system update, make sure your data for ALL your apps is backed up somewhere other than your device (ie to cloud or PC) - in case it goes wrong..