How do I disable rapid charging?
Google Pixel XL allows for slower charging, if a non USB-C adapter is used.

From the support area at Google on the section concerning charging. See the suggestion #2 for your concern:


  Which power adapters and cables to use
  
  
  Pixel phones use USB-C with USB 2.0 power adapters and cables.
  To charge your Pixel phone with a USB-A power adapter, use a USB-C to USB-A cable. This will charge your Pixel phone more slowly than USB-C.
  Other Android cables and power adapters might not work with Pixel phones.
  Pixel phones can't charge wirelessly.