Several years back, WhatsApp introduced new privacy settings that let you change who could see your Profile Picture, About and Last Seen settings. Each of those could be set to allow Everyone, My Contacts or Nobody to see them — not exactly a great level of control. In a new preview, however, WhatsApp introduced a new parameter for "Last Seen" called "My Contacts Except...", allowing you to block specific contacts if you don't want them to see your Last Seen status.
WaBetaInfo has a screenshot taken from iOS as proof, thought it notes that the new feature should eventually appear on both iOS and Android. The parameters will also apply to the Profile Picture and About settings. According to the screenshot, if you block someone's Last Seen setting, you won't be able to see their Last Seen setting in return.
The site notes that the feature is still under development, so it may either appear in a future release or be shelved, provided the post is accurate in the first place. It seems like a pretty logical setting, though, that probably should have been in there already.