[nicw]

"I was surprised to find out that Flickr now requires a Yahoo account to log in. I’m not sure when this was implemented..."

Yahoo usernames were required in 2007: http://news.bbc.co.uk/2/hi/technology/6316761.stm

[ngzhian]

I'm pretty sure from 2007 till 2014 they made some changes to allow users to log on using Facebook/Google accounts, and they reverted in recent months