Main industry for kdb+/q is finance, specifically sell-side or very large buy-side firms.

I wouldn't call it "common", but it's pretty well known.

kdb+/q devs certainly have good job security, heh.

Correct. I'm in trad-fi (traditional finance). The job security is fantastic and the offers are plentiful. The community and support groups are quite small, but very supportive. It just requires a lot more research and effort (compared to JavaScript, Python, Rust, etc.) to find solutions to your problems.