Is this still true? I was in a tournament today where a player was saying that it is not true, based on some 50 degree thing. Where can I read about this to prove it is or isn't.

A character in K14 would not have line of fire to a character in I12. Not because of the degree of the angle (it's a direct diagonal), but because it crosses the intersection of two squares of elevated terrain.
Since elevated terrain blocks line of fire and it crosses the intersection of 2 squares of elevated, there is no least restrictive square, so LoF is blocked.

So if a character is on the corner of elevated terrain does that mean line of fire is blocked through the direct diagonal as well? For example would line of fire be blocked from K15 to I13

No. In the K14 to I12 line of fire, since it is on a direct diagonal, you take the least restrictive of the 2 squares on the corner that do not have LOF drawn through them, in this case I13 and J12. Since both are elevated and would block line of fire, line of fire is blocked.

In the case of the corner like I13, when LOF is drawn from K15, you would use the least restrictive of I14 and J13. Neither of them would block line of fire, so you are good to go. I13 is on the rim.