Google has updated its Google crawler help documentation to specific that the Google Favicon crawler now respects the Googlebot and Googlebot-Image robots rules. Previously, it seemed to just respect its own Google Favicon rules but that was changed or updated a few days ago.
The docs from earlier this year show this:
The revised document shows this:
Dave Smart, where I found this from, said on Twitter "FYI: Looks like the favicon crawler from google now respects Googlebot-Image / Googlebot robots.txt rules, (followed it own, Googlebot favicon, before apparently) so perhaps make sure you're not blocking it accidentally now."
Forum discussion at Twitter.