The benefits of alt tag optimization for traditional SEO has always been a "yo yo" subject for me. Way back in the day (2004 to 2007) I believed there was some benefit to alt tag SEO. However as time went on I saw evidence that the major search engines were no longer considering alt tag SEO as a ranking signal.
However I later had the pleasure to work on a joint project with a high end SEO firm in 2011/2012. My colleagues fully believed that alt tag optimization was still a very important strategy for traditional SEO at that time.
Is there any evidence available that alt tags still help with traditional SEO nowadays? I'm fully aware of the benefits of optimized alt tags and image search. However could optimized alt tags be one of those ranking factors that Google removed due to abuse and later quietly resurrected?