In the update that I did today Icecat is no longer Deterministic. 
I compiled Icecat since there doesn't seem to be any substitutes. After the second time it wrote an error that the two builds were not the same.
My system is this:

Generation 13 Jun 25 2020 16:46:39 (current)
  guix d1a26be
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: d1a26bedcc63362609604ec36c7a5ebee825d140

Marinus