Is the battery in the new Macbook (the one with USB-C) replaceable? It seems that the new battery is glued into the computer. Does this mean that it can't be replaced, or has Apple somehow figured out a way to remove the glue?
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