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robots.txt? NO
"[W]e will eventually have millions of thumbnails in the database. ..." FAQ [community.shrinktheweb.com]
(Wonder how many of those will be shots of custom 403s?;)
(Wonder how many of those will be shots of custom 403s?;)
Probably zero if:
1) you return correct 403 HTTP code as you should (custom txt page is irrelevant)
2) they (and that's very likely) only do screenshots on successfully retrieved pages
