I finally decided to write a custom 404 page since deleting almost 25k of pages in the supplimental index. I put some text and links to my homepage and my sitemap, and I know that you can't put adsense ads on error pages, but what about a search box? I see nothing in the TOS that clearly states that search boxes cannot be used on error pages, just adsense ads, buttons and image ads. Your opinion?
Although I'm not expert with TOS, I think is ok. On one of my sites I'm putting a Google Search Box sitewide, i.e. on ALL pages, including error pages. I'm using that method for +3 years.
I really wouldn't be to worried about that. I do it as well, no problems. However I never formally asked the Adsense team about it.
This is one of those cases where I'd go ahead and ask the Horse. You've got nothing to lose by doing so, and possibly everything to lose by not doing so. I think it would be okay but I'm not the one who would kick you out lol
Why even risk losing your adsense account? Send a quick email to adsense, and be certain before gambling with your income.