HEIC.one uses the following open-source software.

heic-to 1.6.7 (heic-to-1.6.7.next.js)
  https://github.com/hoppergee/heic-to
  Licence: GNU Lesser General Public License v3.0 (LGPL-3.0)
  Includes libheif, also licensed under LGPL-3.0: https://github.com/strukturag/libheif
  The bundled file is the unmodified build published on npm. You may replace it
  with your own compatible build. Licence text: https://www.gnu.org/licenses/lgpl-3.0.txt

gifenc 1.0.3
  https://github.com/mattdesl/gifenc
  Licence: MIT

fflate 0.8.2
  https://github.com/101arrowz/fflate
  Licence: MIT