Terraform Version

0.9.8

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

TLDR: Instead of ~/.terraform.d, configuration should be in the $XDG_CONFIG_HOME/terraform/ and cache (safely-deletable files) should be in $XDG_CACHE_HOME/terraform. If not defined, $XDG_CONFIG_HOME should be defaulted to $HOME/.config and $XDG_CACHE_HOME should be defaulted to $HOME/.cache.
I believe everything currently in .terraform.d is considered cache, therefore there's just $XDG_CACHE_HOME to worry about.

This is pretty standard and helps keep clutter out of the homedir :)