Its on the current patch but i dont understand how this works as i havent been able to do it, ive heard its patched but how is it still done on the current patch?

So it is patched on the current patch of the game. The patch listing on the board is for the arsenal update as a whole which starts with patch 15.

The runs on the board are patch 15b, where as the current most patch is 15d.

in patch 15b every oil splot in water would create its own explosion meaning all you had to do was put a bunch of oil in the water and explode it with proper timing to kill the leviathan instantly.

This was changed in 15c with the patchnotes:

-Underwater gasoline explosions are now sparser
-Underwater gasoline explosions can now only hit an enemy once per continuous explosion string