Bred for War and Malicious Intent have some Phelan in it.
A lot of fans rabidly dislike him for being a traitor, in my experience. Probably why he was kept to a supporting role after his introduction in the trilogy.
He's a creation of Mike Stackpole, and my sense is that there's a strong element of "hands off other people's merchandise" when it came to using him in other books. Which is why you get him in Natural Selection, Malicious Intent, and Grave Covenant--all Stackpole novels--but little elsewhere.
I'm pretty sure those novels were work-for-hire / contracted, so legally the character is part of the IP owned by Topps and licensed to CGL, not property of the author. But I'm also pretty sure that if I pitched a story to "Shrapnel" where I put a bullet in Phelan's head, it would...not be approved. It's an informal, respect thing for Mike and his creation.
On the sourcebook side, you might be interested in "The Clans: Warriors of Kerensky," which is written in-universe by Phelan as a primer on Clan culture, and contains a few interesting asides and back-and-forths between Phelan and his father, Morgan. But even in sourcebooks, you don't hear much of him after Operation SCOUR and nothing in the Dark Age. He's almost certainly dead (unless he's 121 by the time of SF, but let's not go there), but it's unlikely anyone but Mike gets to write the final Phelan story.
(Though I'd certainly like to read that story--it's time to move on to new characters, and better to have an endpoint for one I like a lot, rather than leave him languishing.)
