Is playing a Giant in dungeons and dragons 5E still legal now that Elemental evil is over?

Yes, it is completely legal in the 5th edition handbook that Genasi's are allowed. Giants are unnatural but you may be confused with Goliath, which is also legal in this setting.
It is also up to the DM to decide whether it is allowed in their game so I could be wrong if they play their own rules. But otherwise it's all g :)