Apparently, from Google Books, this construction is not found much in the past; it appears to have become more acceptable since the 1980s, and this is possibly why it doesn't feel as if it really belonged to the language: Google Books — it depends if. It is found increasingly in the past twenty years.

one-gram of the frequency of "it depends if"

There seems to be no rule, but I must say that personally, this construction with "depend" does not feel as idiomatic as "depend on whether". (ref.)

Apparently, from Google Books, this construction is not found much in the past; it appears to have become more acceptable since the 1980s, and this is possibly why it doesn't feel as if it really belonged to the language: Google Books — it depends if. It is found increasingly in the past twenty years.

one-gram of the frequency of "it depends if"

There seems to be no rule, but I must say that, personally, I do not feel this construction with "depend" to be as idiomatic as "it depends on whether". (ref.)