It is one of the most active implementation of Common Lisp (other being Clozure CL) and one of the fastest thanks to its support of optional type hinting (which it had long, long before gradual typing become in vogue recently). With some type hinting, it is possible to generate a code that's almost as fast as C (in certain cases, of course, but still much better than other implementations).
>Nobody seems to have done any serious work in SBCL in over a decade.
Could you please elaborate on what you mean by this?
SBCL releases a new version that contains both enhancements and bugfixes every couple of months, with the latest version (version 1.4.9, released on June 28, 2018, ~5 days ago) came one just one month after the previous version, 1.4.8, so I really don't understand what you mean when you said that no one "seems to have done any serious work in SBCL in over a decade".
You can also see all previous releases and their changlog here: http://www.sbcl.org/all-news.html.
Or did you meant to say that no one use SBCL to do serious work? Well, with some googling, I'm sure you'll be able to find that, while not many, there are companies that use SBCL (and other Common Lisp implementations) in production and other non-trivial works (not to mention various freelancers who use SBCL to put foods on the table). Some of those companies are also involves with the development of SBCL as well.
>Basic things are either completely missing or broken.
Care to list them? One of the strength of SBCL is that the devs are very responsive, especially to bug reports, so I'm sure they would be more than happy to fix the problems that you encountered.