Hi!
Was there some urgent production impact that required doing this with no notice?
Actually we had User-Agent header requirement for ages, it just failed to do what it had to do for a while. Consider this to be a bugfix.
Was any impact analysis done on this?
Yup!
Given Wikipedia's mission, we can't be as casual about rejecting traffic as a commercial site would be. If a commercial site accidentally gets rid of some third-world traffic running behind a shoddy ISP, it's no loss; nobody wants to advertise to them anyhow. But for us, those are the people who gain the most from being able to reach us.
Actually, at the moment this mostly affects crap sites that hot-load data from us to display spamvertisements on hacked sites on internet. I don't know where your 'shoddy ISP' speculation fits in.
Domas