Sicstus Prolog claims that it's used in the transport and logistics industries,
in particular, handling a third of all airline tickets worldwide:

“Most people probably have used SICStus Prolog without knowing it,” says Mats
Carlsson, its lead developer. “One of our customers runs a flight booking system
on SICStus which handles nearly a third of all airline tickets in the world.”

https://www.sics.se/projects/sicstus-prolog-leading-prolog-t...

Quite substantial (for Prolog) applications in Prolog are commonly developed in
the field of Inductive Logic Programming research most of which studies
algorithms that learn Prolog programs from input-output examples. Here's a nice,
recent overview:

Inductive logic programming at 30

https://link.springer.com/article/10.1007%2Fs10994-021-06089...

Historically, you would have found a lot more Prolog (more precisely,
resolution-based logic programming) used in commercial software mostly around
the 1980's and 1990's. Hard to believe today, but there was a time when Prolog
was hyped about as much as deep learning today, as the hot new stuff in AI. The
hype cycle started with the initiation of the Fifth Generation Computer project
in Japan, in 1982:

The Fifth Generation Computer Systems (FGCS) was an initiative by Japan's
Ministry of International Trade and Industry (MITI), begun in 1982, to create
computers using massively parallel computing and logic programming. It was to be
the result of a government/industry research project in Japan during the 1980s.
It aimed to create an "epoch-making computer" with supercomputer-like
performance and to provide a platform for future developments in artificial
intelligence.

https://en.wikipedia.org/wiki/Fifth_generation_computer

The hype cycle picked up steam with a book, published in the US in 1983, written
by Pamela McCorduck and Ed Feigenbaum (Turing Award recipient and the "father of
expert systems") titled: The Fifth Generation: Artificial Intelligence and
Japan's Computer Challenge to the World.

The book awakened a huge FOMO in Western industry and political figures and for
a while everybody was trying to follow on the Japanese' footsteps. Unfortunately
their project failed and probably took logic programming and Prolog with it.
However, for a while at least, you'd have found a whole bunch of quite
substantial Prolog projects being created in industry... and, knowing industry,
it's very likely that quite a few of them are still putt-putt-putting quitely in
some forgotten back-end in a bank, detecting fraud, or in a shipping agency
planning routes etc. People invested lots of money on such systems and they
worked reasonably well (like all software, that is) so nobody would have happily
thrown them out when the AI winter of the '90s hit for good.

I wasn't around back then to see all this happening first hand so I'm
guesstimating about the continued existence of Prolog systems. Anyway, when I
first got interested in Prolog, when I was doing my degree back in 2011, I read
the McCorduck and Feigenbaum book and I was struck by how confident its authors
were that the Fifth Generation Computer project was poised to do to the computer
industry what the Japanese had already done to the home appliances and
automotive industry. I also remember reading a Prolog textbook from the 1990's
(I forget the title- it was not one of the better known ones) whose introduction
chapter opened with a certain exuberance about the usefuleness of knowning
Prolog saying that it is now the language to know and that it's poised to take
over the world, kind of thing.

There's perhaps a lesson to be learned from all this, for people studying more
modern approaches to AI.