It isn’t broken in lts-10.5, it just isn’t in lts-10 at all. Go to https://www.stackage.org/lts-10.5 and search for ghc-mod—it just isn’t currently available.

Unfortunately, the reason for this is that ghc-mod still isn’t updated for GHC 8.2. It’s almost there—a package candidate has been uploaded that works alright—but it hasn’t made it into the LTS yet. There’s a way to build and install the package candidate in a way that will work with stack, which I have done on my personal machine, but honestly I decided it was just too much for this (already overly long) blog post.

Yes, see https://github.com/DanielG/ghc-mod/issues/900