Hm, what does "unmaintained" mean? Sure, long time (more than 10 years)
no updates. Nevertheless, there doesn't seem to be a large number of
requests for changes or corrections that are left without response. You
may say, there are no users, hence no problems. You may also say that
the users are simply quietly satisfied.

And there are a few users, as witnessed by Doug's

  https://mail.haskell.org/pipermail/hugs-users/2018-July/000902.html

and Anthony's

  https://mail.haskell.org/pipermail/hugs-users/2018-July/000900.html

a bit further along in the same thread.

When I started using Haskell in 2003, inspired by my use of Miranda in a
course some 10 years before that, haskell.org had a "Implementation"
button with Hugs, nhc, and GHC (of course) and perhaps more, Helium,
jhc, uhc, lhc come to mind, I am not sure. Even on top of just wishing
the Hugs links repaired, that list would also be high on my wish list.

To be sure, I would never recommend any unmaintained program, such that
Hugs very clearly labels itself on https://www.haskell.org/hugs (the
wording is "no longer in development"), for any "serious" use.
Nevertheless, it seems a waste not to maintain these references to
valuable material, whether for educational, historical or just
entertainment reasons. One valuable development, not particularly recent
but worth mentioning, is Andy Gill's hpc, Haskell Program Coverage, that
took place initially, as I understand, using nhc with Malcolm Wallace
guidance.