In part, a good reason to do this would be that we have indications (at least on English), that a lot more users were looking at the page than would be reasonable for an internal wiki workflow: https://pageviews.wmcloud.org/?project=en.wikipedia.org&platform=all-access&agent=user&redirects=0&range=latest-30&pages=MediaWiki:Bad_image_list

First of all, I decided to check how much traffic the page has across all the wikis. A quick attempt to get that information is at https://public.paws.wmcloud.org/User:Martin_Urbanec/Archive/bad-image-list-pageviews-global.ipynb. Here are the first few rows of the pageviews table:

So, it looks like letting wikis know (perhaps the ones with more than ~50 pageviews/month?) might be a good enough solution, at least for now?

If we do want to make a change, we can do one of the following three things:

A: Add __NOINDEX__ to the page at all wikis where it exists via a bot/script

1. Advantage: Very easy to implement (people make bots all the time)
2. Advantage: Easy and understandable way to revert by community (by reverting the edit)
3. Disadvantage: Making an on-wiki edit by people outside the project's community is always risky (especially at big-ish project; smaller ones usually don't use it I think)
4. Disadvantage: Doesn't scale for newly-created projects

B: Make the change in Wikimedia's config

1. Advantage: Reasonably easy to implement
2. Disadvantage: Works only for Wikimedia

C: Make the change in MediaWiki

1. Advantage: Works for any MediaWiki instance in the world (assuming it gets updated)
2. Disadvantage: Hard(er) to implement properly (can be done as a hack, but I really prefer not introducing more hacks to MW -- we already have a lot of them).