...but why has that been removed? Looks useful to me.

Because it becomes easier to accidentally change the return type of a function.

You might just want to share a video. There were never such inference in Kotlin, so I wonder if there is just some misunderstanding, or just plain bad video.

Unfortunately it's copyrighted. This is it: http://shop.oreilly.com/product/0636920052982.do I think it's bad, too. It also makes another mistake when tries to show a loop: for ( i in 10 .. 1) which doesn't work. This is quite absurd considering it comes from a Kotlin author but that's it.

Those were mistakes. I have no idea now why I made them, but I'll reach out to O'Reilly to see if we can have it highlighted somehow. Apologies.