Is it true that Espeon and Umbreon cannot be caught for the raid bonus challenge?

They changed it.

Also that was never strictly true. While Niantic did not update appropriately at first (shame on them), the mathematics still gave a ~1% chance to catch due to how PoGo treats the value 0. So, while both lazy and shoddy, not death and destruction and the end times and also all puppies vanishing from the earth.