To my knowledge Google never has, and probably never will honor Slayer with a doodle, unfortunately. That doesn't mean Google doesn't appreciate Slayer, as we've seen.
Luckily, someone has imagined what a Slayer-inspired Google doodle might look like. The image was made to honor "Slayer Day," which has been proclaimed by some to be June 6th. You know, 6/6. It made more sense in 2006.
I'm not sure if he created it or not, but Dan Bond posted this to Geeks of Doom back in 2011. We used the image in an article about Google's Halloween Chromebook commercial, which used Slayer's "Raining Blood" back in October, a commercial we revisited today in light of guitarist and songwriter Jeff Hanneman's death.