Malal was dropped due to copyright, retconned and eventually became Malice
Malal popped up in the 80s, but due to disagreement between GW and the pair of authors who actually created him, it was not clear who owned the copyright so he was dropped.
Use of Malal in further Games Workshop productions ceased around 1988, the same year the first of the two Realm of Chaos background books was published. Malal is not referred to or mentioned at all in these products. There was also an uncertainty as to who actually owned the rights to the concept of Malal -- the comic's authors or Games Workshop.
But, there are still references to him scattered throughout 40k and Fantasy, as recent as 2002. But it seems he has been retconned entirely and is now known as Malice.
One of the examples was a Chaos Space Marine of a Renegade Chapter entitled the "Sons of Malice". The colours used for this Chapter were the bisecting black/white design of Malal's original mark, and the word "Malice" is not too dissimilar from "Malal"; in essence, Malal has been returned to the Warhammer 40,000 universe as the Renegade Chaos God Malice, the Hierarch of Anarchy and Terror. The word "Malal" also means "Malice" in several East Indian languages. More information on the Sons of Malice came in Games Workshop's monthly publication White Dwarf 303 (issue 302 in the U.S.). The article mentioned that the Sons of Malice were exiled from the Imperium for a set of disgusting rituals that were reported to include cannibalism and that they fought in complete silence. The ultimate patron deity of the ritual was never revealed, though it can be surmised that it was Malice. At the end of the article it was specifically mentioned that the Sons of Malice were noted to fight with ferocity against other followers of Chaos.
I actually always thought Be'lakor and Malal were the same entity, turns out I was wrong.