I had suspected this possibility, but never heard anything. Some googling found this 2018 topic by Klasodeth, a friend of Lucius.
https://forums.dfworkshop.net/viewtopic.php?f=8&t=940
This is long after he discontinued work of the project, so I don't think death contributed to the lack of updates.Lucius has been doing well. Life has been busy for him over the last few years, though it's settled down a bit these days. He's still expressed interest in continuing work on the XL Engine, but I get the distinct impression he's recovering from the years of having incredible demands on his time. There's no telling when he'll finally decide to return to XL Engine development, but when he does he'll be focusing on the decompiler, which should help result in more accurate gameplay reproduction even for other people's existing projects. But of course, that's at some indeterminate point in the future, and I can't blame anyone for concluding that it may never happen.