There's no actual legal challenge on the go. It's just a thought or a consideration or an analysis. The essence is that the legislation clearly draws a distinction between artists whose works predate 1988 and artists whose works don't. That on its face may not be considered discriminatory, but the fact of the matter is that it is highly likely and it's logical that works created before 1988 are more likely going to be works of older visual artists.
As a result, this is an arbitrary line. Lawyers don't like to be absolute like this, but it's probably one of the most arbitrary lines you can draw. There's a date that's fixed in 1988 and if you created something before that, you're out of luck. Most of those people are likely to be older. Or it will be their estates that are losing out on that benefit as well.