Is the battery in the new Macbook (the one with USB-C) replaceable?
The battery in the new MacBook is replaceable but Apple recommends that you bring it to them for servicing and replacement, if necessary.  Here's what Apple says about the battery:


  MacBook, MacBook Air, and MacBook Pro computers with built-in batteries should only have their batteries replaced by an Apple Authorized Service Provider or Apple Retail Store. Attempting to replace a built-in battery yourself could damage your equipment, and such damage is not covered by warranty.
  
  These Mac notebooks have built-in batteries:
  
  
  MacBook (13-inch, Late 2009) and later
  All MacBook Air computers
  All MacBook Pro computers with Retina display
  MacBook Pro (13-inch, Mid 2009) and later; MacBook Pro (15-inch, Mid 2009) and later