The King's Forest slayer XP is a real loss if you ETR after finishing it on an earlier life. With usual XP bonuses running and with cracking a 30% XP pot for the slayers, you can get about one-eighth of the total XP needed to hit 30 again (you can get about 1,060,000 from the King's Forest, out of the 8,250,000 needed to hit 30). And you can get it in about 75-80 minutes if you are willing to buy a major slayer pot.
There's no "story" reason why a character wouldn't gain experience by exploring the King's Forest. They are back to being level 20, so they should get the XP.
Just a suggestion.